Move the CoreStage::Startup to a seperate StartupSchedule label (#2434)

# Objective

- `CoreStage::Startup` is unique in the `CoreStage` enum, in that it represents a `Schedule` and not a `SystemStage`.
- This can lead to confusion about how `CoreStage::Startup` and the `StartupStage` enum are related.
- Beginners sometimes try `.add_system_to_stage(CoreStage::Startup, setup.system())` instead of `.add_startup_system(setup.system())`, which causes a Panic:
```
thread 'main' panicked at 'Stage 'Startup' does not exist or is not a SystemStage', crates\bevy_ecs\src\schedule\mod.rs:153:13
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b\/library\std\src\panicking.rs:493
   1: std::panicking::begin_panic_fmt
             at /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b\/library\std\src\panicking.rs:435
   2: bevy_ecs::schedule::{{impl}}::add_system_to_stage::stage_not_found
             at .\crates\bevy_ecs\src\schedule\mod.rs:153
   3: bevy_ecs::schedule::{{impl}}::add_system_to_stage::{{closure}}<tuple<bevy_ecs::system::function_system::IsFunctionSystem, tuple<bevy_ecs::system::commands::Commands, bevy_ecs::change_detection::ResMut<bevy_asset::assets::Assets<bevy_render::mesh::mesh::Me
             at .\crates\bevy_ecs\src\schedule\mod.rs:161
   4: core::option::Option<mut bevy_ecs::schedule::stage::SystemStage*>::unwrap_or_else<mut bevy_ecs::schedule::stage::SystemStage*,closure-0>
             at C:\Users\scher\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\option.rs:427
   5: bevy_ecs::schedule::Schedule::add_system_to_stage<tuple<bevy_ecs::system::function_system::IsFunctionSystem, tuple<bevy_ecs::system::commands::Commands, bevy_ecs::change_detection::ResMut<bevy_asset::assets::Assets<bevy_render::mesh::mesh::Mesh>>, bevy_ec
             at .\crates\bevy_ecs\src\schedule\mod.rs:159
   6: bevy_app::app_builder::AppBuilder::add_system_to_stage<tuple<bevy_ecs::system::function_system::IsFunctionSystem, tuple<bevy_ecs::system::commands::Commands, bevy_ecs::change_detection::ResMut<bevy_asset::assets::Assets<bevy_render::mesh::mesh::Mesh>>, be
             at .\crates\bevy_app\src\app_builder.rs:196
   7: 3d_scene::main
             at .\examples\3d\3d_scene.rs:4
   8: core::ops::function::FnOnce::call_once<fn(),tuple<>>
             at C:\Users\scher\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:227
```

## Solution

- Replace the `CoreStage::Startup` Label with the new `StartupSchedule` unit type.


Resolves #2229

crates/bevy_app/src/app.rs

@@ -1,4 +1,6 @@
-use crate::{CoreStage, Events, Plugin, PluginGroup, PluginGroupBuilder, StartupStage};
+use crate::{
+    CoreStage, Events, Plugin, PluginGroup, PluginGroupBuilder, StartupSchedule, StartupStage,
+};
 pub use bevy_derive::AppLabel;
 use bevy_ecs::{
     prelude::{FromWorld, IntoExclusiveSystem},
@@ -207,7 +209,7 @@ impl App {
     /// ```
     pub fn add_startup_stage<S: Stage>(&mut self, label: impl StageLabel, stage: S) -> &mut Self {
         self.schedule
-            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
+            .stage(StartupSchedule, |schedule: &mut Schedule| {
                 schedule.add_stage(label, stage)
             });
         self
@@ -238,7 +240,7 @@ impl App {
         stage: S,
     ) -> &mut Self {
         self.schedule
-            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
+            .stage(StartupSchedule, |schedule: &mut Schedule| {
                 schedule.add_stage_after(target, label, stage)
             });
         self
@@ -269,7 +271,7 @@ impl App {
         stage: S,
     ) -> &mut Self {
         self.schedule
-            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
+            .stage(StartupSchedule, |schedule: &mut Schedule| {
                 schedule.add_stage_before(target, label, stage)
             });
         self
@@ -483,7 +485,7 @@ impl App {
         system: impl IntoSystemDescriptor<Params>,
     ) -> &mut Self {
         self.schedule
-            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
+            .stage(StartupSchedule, |schedule: &mut Schedule| {
                 schedule.add_system_to_stage(stage_label, system)
             });
         self
@@ -519,7 +521,7 @@ impl App {
         system_set: SystemSet,
     ) -> &mut Self {
         self.schedule
-            .stage(CoreStage::Startup, |schedule: &mut Schedule| {
+            .stage(StartupSchedule, |schedule: &mut Schedule| {
                 schedule.add_system_set_to_stage(stage_label, system_set)
             });
         self
@@ -588,7 +590,7 @@ impl App {
     pub fn add_default_stages(&mut self) -> &mut Self {
         self.add_stage(CoreStage::First, SystemStage::parallel())
             .add_stage(
-                CoreStage::Startup,
+                StartupSchedule,
                 Schedule::default()
                     .with_run_criteria(RunOnce::default())
                     .with_stage(StartupStage::PreStartup, SystemStage::parallel())

crates/bevy_app/src/lib.rs

@@ -20,7 +20,9 @@ pub use schedule_runner::*;
 #[allow(missing_docs)]
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{app::App, CoreStage, DynamicPlugin, Plugin, PluginGroup, StartupStage};
+    pub use crate::{
+        app::App, CoreStage, DynamicPlugin, Plugin, PluginGroup, StartupSchedule, StartupStage,
+    };
 }
 
 use bevy_ecs::schedule::StageLabel;
@@ -30,11 +32,6 @@ use bevy_ecs::schedule::StageLabel;
 /// The relative stages are added by [`App::add_default_stages`].
 #[derive(Debug, Hash, PartialEq, Eq, Clone, StageLabel)]
 pub enum CoreStage {
-    /// Runs only once at the beginning of the app.
-    ///
-    /// Consists of the sub-stages defined in [`StartupStage`]. Systems added here are
-    /// referred to as "startup systems".
-    Startup,
     /// Name of app stage that runs before all other app stages
     First,
     /// Name of app stage responsible for performing setup before an update. Runs before UPDATE.
@@ -47,6 +44,15 @@ pub enum CoreStage {
     /// Name of app stage that runs after all other app stages
     Last,
 }
+
+/// The label for the Startup [`Schedule`](bevy_ecs::schedule::Schedule),
+/// which runs once at the beginning of the app.
+///
+/// When targeting a [`Stage`](bevy_ecs::schedule::Stage) inside this Schedule,
+/// you need to use [`StartupStage`] instead.
+#[derive(Debug, Hash, PartialEq, Eq, Clone, StageLabel)]
+pub struct StartupSchedule;
+
 /// The names of the default App startup stages
 #[derive(Debug, Hash, PartialEq, Eq, Clone, StageLabel)]
 pub enum StartupStage {