Allow the editing of startup schedules (#10969)

# Objective

Fixes #10968 

## Solution

Pull startup schedules from a list of `ScheduleLabel`s in the same way
the update schedules are handled.

---

## Changelog

- Added `MainScheduleOrder::startup_labels` to allow the editing of the
startup schedule order.

## Migration Guide

- Added a new field to `MainScheduleOrder`, `startup_labels`, for
editing the startup schedule order.

crates/bevy_app/src/main_schedule.rs

@@ -118,8 +118,10 @@ pub struct Last;
 /// their order.
 #[derive(Resource, Debug)]
 pub struct MainScheduleOrder {
- /// The labels to run for the [`Main`] schedule (in the order they will be run).
+ /// The labels to run for the main phase of the [`Main`] schedule (in the order they will be run).
  pub labels: Vec<InternedScheduleLabel>,
+ /// The labels to run for the startup phase of the [`Main`] schedule (in the order they will be run).
+ pub startup_labels: Vec<InternedScheduleLabel>,
 }
 
 impl Default for MainScheduleOrder {
@@ -135,12 +137,13 @@ impl Default for MainScheduleOrder {
  PostUpdate.intern(),
  Last.intern(),
  ],
+ startup_labels: vec![PreStartup.intern(), Startup.intern(), PostStartup.intern()],
  }
  }
 }
 
 impl MainScheduleOrder {
- /// Adds the given `schedule` after the `after` schedule
+ /// Adds the given `schedule` after the `after` schedule in the main list of schedules.
  pub fn insert_after(&mut self, after: impl ScheduleLabel, schedule: impl ScheduleLabel) {
  let index = self
  .labels
@@ -149,15 +152,31 @@ impl MainScheduleOrder {
  .unwrap_or_else(|| panic!("Expected {after:?} to exist"));
  self.labels.insert(index + 1, schedule.intern());
  }
+
+ /// Adds the given `schedule` after the `after` schedule in the list of startup schedules.
+ pub fn insert_startup_after(
+ &mut self,
+ after: impl ScheduleLabel,
+ schedule: impl ScheduleLabel,
+ ) {
+ let index = self
+ .startup_labels
+ .iter()
+ .position(|current| (**current).eq(&after))
+ .unwrap_or_else(|| panic!("Expected {after:?} to exist"));
+ self.startup_labels.insert(index + 1, schedule.intern());
+ }
 }
 
 impl Main {
  /// A system that runs the "main schedule"
  pub fn run_main(world: &mut World, mut run_at_least_once: Local<bool>) {
  if !*run_at_least_once {
- let _ = world.try_run_schedule(PreStartup);
- let _ = world.try_run_schedule(Startup);
- let _ = world.try_run_schedule(PostStartup);
+ world.resource_scope(|world, order: Mut<MainScheduleOrder>| {
+ for &label in &order.startup_labels {
+ let _ = world.try_run_schedule(label);
+ }
+ });
  *run_at_least_once = true;
  }