Introduce public World::register_dynamic_bundle method

[urben1680]

Registering dynamic bundles was not possible for the user yet.

It is alone not very useful though as there are no methods to clone, move or remove components via a BundleId. This could be a follow-up work if this PR is approved and such a third (besides T: Bundle and ComponentId(s)) API for structural operation is desired. I certainly would use a hypothetical EntityClonerBuilder::allow_by_bundle_id.

I personally still would like this register method because I have a Bundles-like custom data structure and I would like to not reinvent the wheel. Then instead of having boxed ComponentId slices in my collection I could look up explicit and required components there.

For reference scroll up to the typed version right above the new one.

[urben1680]

If this is too little to add, I can also extend EntityClonerBuilder and add remove_by_bundle_id to EntityCommands/EntityWorldMut.