[web] switch to SemanticsAction.focus (attempt 3)

[yjbanov]

This relands https://github.com/flutter/engine/pull/53679.

Stop using SemanticsAction.didGain/LoseAccessibilityFocus on the web, start using SemanticsAction.focus. This is because on the web, a11y focus is not observable, only input focus is. Sending SemanticsAction.focus will guarantee that the framework move focus to the respective widget. There currently is no "unfocus" signal, because it seems to be already covered: either another widget gains focus, or an HTML DOM element outside the Flutter view does, both of which have their respective signals already.

More details in the discussion in the issue https://github.com/flutter/flutter/issues/83809.

Fixes https://github.com/flutter/flutter/issues/83809 Fixes https://github.com/flutter/flutter/issues/148285 Fixes https://github.com/flutter/flutter/issues/143337

[flutter-dashboard[bot]]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.