Add bevy_asset

[geirsagberg]

Latest bevy_main failed because bevy_asset seems to have been moved into an explicit feature:

error[E0432]: unresolved import `bevy::asset`                                                                                                                                  
 --> C:\Users\Geir\.cargo\git\checkouts\bevy_asset_loader-61e45c7c12c9b28c\2e9f2a5\bevy_asset_loader\src\asset_collection.rs:3:11
  |
3 | use bevy::asset::HandleUntyped;
  |           ^^^^^ could not find `asset` in `bevy`

error[E0432]: unresolved import `bevy::asset`
 --> C:\Users\Geir\.cargo\git\checkouts\bevy_asset_loader-61e45c7c12c9b28c\2e9f2a5\bevy_asset_loader\src\dynamic_asset.rs:5:11
  |
5 | use bevy::asset::{AssetServer, HandleUntyped};
  |           ^^^^^ could not find `asset` in `bevy`

With adding the feature as default, the project builds.

[NiklasEi]

CI is red, because bevy_common_assets was already updated to Bevy 0.8 I'll take care of it