Anim: bone collections, split ARMATURE_OT_collection_assign in two

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.

scripts/startup/bl_ui/space_view3d.py

@@ -4117,7 +4117,7 @@ class VIEW3D_MT_bone_collections(Menu):
 
         layout.separator()
 
-        props = layout.operator("armature.collection_assign",
+        props = layout.operator("armature.collection_create_and_assign",
                                 text="Assign to New Collection")
         props.name = "New Collection"