Adds very preliminary support for Windows and Linux plugins:
- Adds those platforms to the new plugin schema, initially supporting just a plugin class.
- Adds C++ plugin registrant generation for any Windows or Linux plugins found.

This doesn't have yet have any build tooling for either platform, so anyone using the generated registrant still needs to do manual build configuration. This reduces the manual work, however, and creates a starting point for future tooling work.

As with all Windows and Linux work at this time, this is not final, and subject to change without warning in the future (e.g., Windows could potentially switch to a C# interface, or
'linux' may change to 'gtk' or 'linux_gtk' in pubspec.yaml).

* 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