Split the `armature.collection_assign` operator in two. Before, the
operator could do two things: assign selected bones to the active/named
bone collection, or create a new bone collection with the given name.
This is now split up, where `armature.collection_assign` only assigns to
existing bone collections, and `armature.collection_create_and_assign`
always creates a new bone collection.
This makes the purpose of each operator clearer & more predictable.
b70ebf96e7