Provide a new user.shortcut_set cache context

MR created to try to address this shortcut caching improvement.

Added some simple typehints nothing major.

I did purposely not add one to getLabel as the docs say it returns a string but translatable markup seems more correct. But definitely out of scope.

Two questions on the MR.

I opened 📌 Replace the shortcut_set_users table with a base field on users Active , if we had that, we wouldn't need the extra query, we could look up the shortcut set from the user object which we'll have anyway. I think this probably needs to be postponed on that issue.