extensions: Add static defineTranslationFunctions() method

extensions: Add static defineTranslationFunctions() method

The method can be used to define a set of gettext functions that
call the corresponding method of an extension.

Those functions are very similar to the gettext functions we are
exporting, except that:

 - looking up the extension is delegated to the
   Extension/Preferences class
 - it is possible to avoid examining the stack
   when called with `import.meta.url`

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2838>