Adds "hot reloading" of internal assets, which is normally not possible because they are loaded using `include_str` / direct Asset collection access.
This is accomplished via the following:
* Add a new `debug_asset_server` feature flag
* When that feature flag is enabled, create a second App with a second AssetServer that points to a configured location (by default the `crates` folder). Plugins that want to add hot reloading support for their assets can call the new `app.add_debug_asset::<T>()` and `app.init_debug_asset_loader::<T>()` functions.
* Load "internal" assets using the new `load_internal_asset` macro. By default this is identical to the current "include_str + register in asset collection" approach. But if the `debug_asset_server` feature flag is enabled, it will also load the asset dynamically in the debug asset server using the file path. It will then set up a correlation between the "debug asset" and the "actual asset" by listening for asset change events.
This is an alternative to #3673. The goal was to keep the boilerplate and features flags to a minimum for bevy plugin authors, and allow them to home their shaders near relevant code.
This is a draft because I haven't done _any_ quality control on this yet. I'll probably rename things and remove a bunch of unwraps. I just got it working and wanted to use it to start a conversation.
Fixes#3660
98938a8555