Add importmap_watches setting for the import map cache sweeper - #9153

Adds config.importmap_watches, the directories whose JavaScript changes clear the ActiveAdmin.importmap cache in development and test. It starts with Active Admin's own app/javascript.

Importmap::Map#cache_sweeper(watches:) builds a new watcher on each call, so an engine passing its own directory replaced Active Admin's. Now engines append to the list, and Active Admin builds the sweeper once in after_initialize. That also lets the host append from config/initializers/active_admin.rb, which runs after engine initializers.

Things a reviewer may ask:

• A plugin that still calls cache_sweeper(watches:) itself loses its watcher to the one built in after_initialize. It should append to importmap_watches instead.
• Once the sweeper is built, the list is frozen, so appending to it raises a FrozenError. The setting's comment and the docs say to append from an initializer. A late append, for example from config.to_prepare, would otherwise never be watched.
• Rails' file watcher skips directories under Gem.path, so only path gems and in-app engines benefit.
• Application#settings dups the default per application, as Add javascript_modules setting to import modules on every page #9148 does for javascript_modules. The two PRs conflict on that line; the resolution keeps both.

Part of #9147. Specs cover the default, the per-application copy, the error on a late append, and a cache clear after editing a file in an added directory.