Customize panic handling

[Bromeon]

When a panic in gdext or Rust code happens, it is currently caught and displayed as a Godot error and on the command line.

Users may want to customize this. Behaviors could be:

• Godot/console message of varying verbosity levels (message -> context -> stacktrace)
• Instantly crash (to trigger a breakpoint)
• Trigger a breakpoint without crashing (only possible with nightly breakpoint or platform specific crates/code)
• Custom handler (would need to ensure thread safety)

[Bromeon]

A lot of the panicking operations now have a fallible overload, see #634.

It can still make sense to customize the behavior when actual panics happen. One idea:

unsafe impl ExtensionLibrary for MyLibrary {
    fn panic_behavior() -> PanicBehavior { ... }
}

enum PanicBehavior {
    Default, // print to stdout + Godot
    CrashEngine,
    Breakpoint, // nightly-only
    Custom {
        callback: Box<dyn FnMut(...)>, // parameter could be message or panic object
    }
}