extensionUtils: Split out extension convenience functions

extensionUtils: Split out extension convenience functions

ExtensionUtils was originally used for shared functions between
the extension system and the (old) prefs-tool, but then gained
useful API meant for extensions themselves.

It's a bit weird to mix the two, so split out the extension convenience
API into a separate module.

We will soon split up the module further, and add specific "flavors"
for extensions and preferences, with the current code providing a
shared base for both.

That should explain both the new location and the odd module name. :-)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2837>