Bevy importer

[kayhhh]

Create a custom gltf -> Bevy importer using gltf_kun. I don't like doing this since Bevy already has a gltf importer, but am not sure of a good way to use it while supporting custom extensions. There is a PR to expose custom extensions from the Bevy importer which may be one way to solve the problem, but even so that will not be usable for some time unless we start using a fork.

[kayhhh]

Things are mostly working 👍 at least for the existing gltf_kun features, still missing materials and such.

Sadly for importing you cannot include bevy_gltf in your app, as each file extension can only have one asset loader. This makes it a bit annoying to work with as bevy_gltf is included in the default features / DefaultPlugins of Bevy. Also makes it hard / impossible for the plugin to be used in the background by bevy libraries as many people will be using bevy_gltf in their app.

However that only applies to .gltf and .glb loading. Other formats not supported already by Bevy like .glxf should still work even with bevy_gltf added to the app (there may be logical errors to account for such as the name-based resolution of assets, but that is solveable, the loader is still run).