ExtensionLibrary::on_main_loop_*: merge into new on_stage_init/deinit API

[Bromeon]

Iterates on the API design for main-loop callbacks, introduced in #1313.
Quick comparison:

(0): Status quo (master)

Separate methods for each callback. Direct C mapping.

trait ExtensionLibrary {
    fn on_main_loop_startup() {}
    fn on_main_loop_frame() {}
    fn on_main_loop_shutdown() {}
}

(1): Group main-loop together

See first commit 74f9a01.

enum MainLoopEvent {
    Init,
    Frame,
    Deinit,
}

trait ExtensionLibrary {
    fn on_main_loop_event(event: MainLoopEvent) {}
}

Smaller API in ExtensionLibrary and fully independent of existing InitLevel, at the cost of enum + case differentiation even for Frame.

(2): Group init level/stage together

See second commit a1d9adc.

// Like InitLevel with added MainLoop
enum InitStage {
    Core,
    Servers,
    Scene,
    Editor,
    MainLoop,
}

trait ExtensionLibrary {
    fn on_stage_init(stage: InitStage) {}
    fn on_stage_deinit(stage: InitStage) {}
    fn on_main_loop_frame() {}
}

Consistently treats all initialization stages (main loop has strict ordering > init levels). However, both InitLevel + InitStage will need to coexist, as the latter cannot be used for e.g. a class' init level.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1380

[TitanNano]

I would prefer Solution 2. It feels most consistent with what we already have and does not introduce two extra functions.