[Flutter Web] Create proof of concept wrappers for focus management

[tugorez]

Use case

As part of #137443 we're adding new capabilities to the web engine to report view focus changes.

Proposal

Until the framework is able to properly handle these signals we should create wrappers that will help us validate these changes.

For example https://github.com/tugorez/flutter_focus/blob/main/lib/focusable_app.dart works with full mode and embedded apps by wrapping them like https://github.com/tugorez/flutter_focus/blob/main/lib/main_with_focus_handlers.dart

[huycozy]

I guess this is mainly implemented on the framework side so will also add framework labels.

[ditman]

Alternatively, maybe help this land?

• Factor out RawView, make View listen to engine generated view focus events #143259