Presently can be provided by php-developers (the small and - for good reason - tightly controlled community), by admins for all pages, or for individual users (self-created or as personally enabled widgets). For all pages is often too dangerous to become a basis for agile improvements, for one self is not very motivating.
I propose to add a tag to load admin-provided scripts from the Mediawiki namespace. It could be
but should always be limited to the Mediawiki namespace.