ruff server reads from a configuration TOML file in the user config…

…uration directory if no local configuration exists (#11225)

## Summary

Fixes #11158.

A settings file in the ruff user configuration directory will be used as
a configuration fallback, if it exists.

## Test Plan

Create a `pyproject.toml` or `ruff.toml` configuration file in the ruff
user configuration directory.

* On Linux, that will be `$XDG_CONFIG_HOME/ruff/` or `$HOME/.config`
* On macOS, that will be `$HOME/Library/Application Support`
* On Windows, that will be `{FOLDERID_LocalAppData}`

Then, open a file inside of a workspace with no configuration. The
settings in the user configuration file should be used.

crates/ruff_server/src/session/workspace/ruff_settings.rs

@@ -4,7 +4,7 @@ use ruff_linter::{
 };
 use ruff_workspace::{
     configuration::{Configuration, FormatConfiguration, LintConfiguration, RuleSelection},
-    pyproject::settings_toml,
+    pyproject::{find_user_settings_toml, settings_toml},
     resolver::{ConfigurationTransformer, Relativity},
 };
 use std::{
@@ -80,9 +80,23 @@ impl RuffSettingsIndex {
             }
         }
 
+        let fallback = find_user_settings_toml()
+            .and_then(|user_settings| {
+                ruff_workspace::resolver::resolve_root_settings(
+                    &user_settings,
+                    Relativity::Cwd,
+                    &EditorConfigurationTransformer(editor_settings, root),
+                )
+                .ok()
+            })
+            .unwrap_or_default();
+
         Self {
             index,
-            fallback: Arc::default(),
+            fallback: Arc::new(RuffSettings {
+                formatter: fallback.formatter,
+                linter: fallback.linter,
+            }),
         }
     }