Option for the plugin to skip setting an initial state

[will-hart]

Background

While integrating this into a template which used bevy_asset_loader I found that there was a little bit of friction as both plugins wanted to manage state. In particular, the leafwing ManifestPlugin automatically sets the game to AssetLoadingState::LOADING when the plugin is added. I still want leafwing_manifest to manage manifest processing states, but I don't want it to control my application setup which seems to be the current behaviour.

I think this is different from automatically_advance_states, in that Having this allows me to explicitly control when I hand over control of the application state to leafwing_manifest by triggering the right state transition. (Its definitely possible I've missed a more obvious approach😁)

Changes

The default behaviour is unchanged. The changes are:

1. add a set_initial_state flag to ManifestPlugin, which defaults to true,
2. in ManifestPlugin::build check set_initial_state is true before calling app.insert_state(S::LOADING).
3. made the Default impl for ManifestPlugin generic across AssetLoadingStates, which might resolve ManifestPlugin's Default impl is specific to SimpleAssetState #46 ?
4. added an example of how to integrate with "preloaded" / preprocessed assets.