Make SubStates work

[mgi388]

Prior to this change, Bevy SubStates do not work. Running the new example gives this panic:

❯ cargo run --example sub_state
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.18s
     Running `target/debug/examples/sub_state`
2024-10-21T11:30:32.578903Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 14.4.1 ", kernel: "23.4.0", cpu: "Apple M1 Max", core_count: "10", memory: "64.0 GiB" }
2024-10-21T11:30:32.681630Z  INFO bevy_render::renderer: AdapterInfo { name: "Apple M1 Max", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }
2024-10-21T11:30:33.216368Z  INFO bevy_winit::system: Creating new window "App" (Entity { index: 0, generation: 1 })
thread 'main' panicked at /me/bevy_asset_loader/bevy_asset_loader/src/loading_state/systems.rs:208:23:
Requested resource bevy_state::state::resources::State<sub_state::MainMenuState> does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_event`,
                and can be added by plugins.
stack backtrace:
...

With my fix, the new example works:

❯ cargo run --example sub_state
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.18s
     Running `target/debug/examples/sub_state`
2024-10-21T11:48:12.776700Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 14.4.1 ", kernel: "23.4.0", cpu: "Apple M1 Max", core_count: "10", memory: "64.0 GiB" }
2024-10-21T11:48:12.871491Z  INFO bevy_render::renderer: AdapterInfo { name: "Apple M1 Max", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }
2024-10-21T11:48:13.397125Z  INFO bevy_winit::system: Creating new window "App" (Entity { index: 0, generation: 1 })
2024-10-21T11:48:13.430028Z  INFO sub_state: Booting...press space to finish booting
2024-10-21T11:48:14.211241Z ERROR bevy_asset::server: Path not found: /me/bevy_asset_loader/bevy_asset_loader/assets/non-existing-file.ogg
2024-10-21T11:48:14.227443Z  INFO sub_state: As expected, bevy_asset_loader switched to the failure state
2024-10-21T11:48:14.227467Z  INFO sub_state: Quitting the application...

I also added a test to prove it works.

The change seems sensible to me: We shouldn't look up the State<S> resource unless there is at least some next state, so it's just moved down. The reason this can't be at the top is that when you add a sub state in Bevy using app.add_sub_state::<S>(), Bevy doesn't seem to initialise the State<S> resource, not sure why.

[mgi388]

Fixed the lint issue.