Enum support in SettingsGroup derive macro, implemented key attribute

[mpowell90]

Objective

Implements part of #23302

Fixes: #23722

For background on bevy settings see the initial PR:
#23034

Solution

Supports unit-like Enum resources:

#[derive(Resource, SettingsGroup, Reflect, Debug, Default)]
#[reflect(Resource, SettingsGroup, Default)]
enum CounterRefreshRateSettings {
    #[default]
    Slow,
    Fast,
}

Assuming the above is initialised as CounterRefreshRateSettings::Slow, results in:

[counter_refresh_rate_settings]
counter_refresh_rate_settings = "Slow"

Setting the group attribute works the same as structs:

#[settings_group(group = "counter_settings")]

Results in:

[counter_settings]
counter_refresh_rate_settings = "Slow"

This PR adds a key attribute which can only be used on enums (for now), otherwise a compile-time error is thrown:

#[settings_group(key = "refresh_rate")]

Results in:

[counter_refresh_rate_settings]
refresh_rate = "Slow"

Testing

• Added tests that checks merging enums into groups and round trip serialisation.
• Added a "cheat sheet" for the derive syntax, immitating other derive macros in the file.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23719

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[viridia]

Awesome work!

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23719

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[mpowell90]

FAO reviewers: i've added the fix for #23722 to this PR as it touches the same code. I can split out into a separate PR if preferred.

[kfc35]

Just some doc refinements and some code clean-ups that I’d like to see get done (specifically that removal of the redundant code block)

Also @DavidCrossman ’s comment is valid

Feel free to re-request after you’ve addressed everything!

[kfc35]

The wasm build failing is unrelated to the PR, but is a legitimate issue that needs to be resolved before CI can pass (probably should be a separate PR)

Copied from the logs for convenience:

error[E0432]: unresolved import `crate::prefs_file`
 --> crates/bevy_settings/src/store_wasm.rs:1:12
  |
1 | use crate::prefs_file::serialize_table;
  |            ^^^^^^^^^^ could not find `prefs_file` in the crate root

error[E0432]: unresolved imports `crate::prefs`, `crate::PreferencesFile`, `crate::PreferencesFileContent`
 --> crates/bevy_settings/src/store_wasm.rs:2:13
  |
2 | use crate::{prefs::PreferencesStore, PreferencesFile, PreferencesFileContent};
  |             ^^^^^                    ^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^ no `PreferencesFileContent` in the root
  |             |                        |
  |             |                        no `PreferencesFile` in the root
  |             could not find `prefs` in the crate root
  |
help: a similar name exists in the module
  |
2 - use crate::{prefs::PreferencesStore, PreferencesFile, PreferencesFileContent};
2 + use crate::{prefs::PreferencesStore, PreferencesStore, PreferencesFileContent};
  |
help: a similar name exists in the module
  |
2 - use crate::{prefs::PreferencesStore, PreferencesFile, PreferencesFileContent};
2 + use crate::{prefs::PreferencesStore, PreferencesFile, PreferenceFileManifest};
  |

For more information about this error, try `rustc --explain E0432`.
error: could not compile `bevy_settings` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
Error: Process completed with exit code 101.

[kfc35]

Now that #23779 is merged, I think this just needs main merged into it, and then it should pass CI