Rethink naming and rules for attributes

[NiklasEi]

A few issues with the current naming and attribute rules:

1. "Collection" is used for structs implementing AssetCollection and for folders and files as attributes
2. It's not clear when attributes like collection or standard_material need to be set for dynamic asset fields (first yes, second no)
3. Need to clarify "dynamic" more
   • What's a dynamic asset collection? The struct or the ron file?
   • What to call the ron files? Currently, "dynamic asset collection" or "dynamic asset collection file" to distinguish from the first.

For assets known at compile-time, attributes need to distinguish between:

1. Handle<T>
2. Texture atlas
3. Standard material
4. folder
5. typed folder
6. collection
7. typed collection

For assets resolved at run-time (dynamic assets), attributes need to distinguish the following cases at compile-time:

1. Handle<T> (including texture atlas and standard material)
2. Option<Handle<T>> (only supported for dynamic assets)
3. folder/collection
4. typed folder/collection

See the full_collection and full_dynamic_collection examples for current code.