Add a context menu dedicated to asset operations to the Asset Browser.
There are two separate context menus to keep things separated well and
avoid confusing if-else logic (similar to D12057 & D12059). Their polls
make sure they are displayed for the right contexts only.
Also (to be committed as followup cleanup): Remove now unused special
handling for assets in file delete operator.
Differential Revision: https://developer.blender.org/D12062
3ff5d8f719