Automatically generated registrants for web plugins (#39628)

* WIP on web plugin registry

* WIP on registering plugins

* WIP on web plugin registration

* Only generate `package:flutter_web_plugins` imports if plugins are
defined

* Add parsing test

* Add documentation

* Fix analyzer warnings

* add license headers

* Add tests for package:flutter_web_plugins

* Run `flutter update-packages --force-upgrade`

* Fix analyzer errors

* Fix analyzer error in test

* Update copyright and remove flutter SDK constraints

* Enable tests since engine has rolled

* add flutter_web_plugins tests to bots

* Create an empty .packages file for WebFs test