Writing Flow: Unset typing flag if Escape pressed

[aduth]

Extracted from #10699

This pull request seeks to resolve an issue where the contextual block toolbar is not accessible by keyboard when typing, except by artificially creating a selection for the mere purpose of deactivating the "is typing" application state flag. It does so by canceling the typing mode when the user presses Escape.

Implementation notes:

There are still some undesirable behaviors, of which this is not intended to cover, though I will create issues for:

• There is special handling of a new paragraph block to consider it as "not yet a paragraph block", thus requiring some typing to occur before Escape will work. Ideally the behavior is made consistent so that Escape will always activate the toolbar.
  • Related: Block actions not highly visible #7177, In-Page Inserter Revisions #10519, Confusing + button behavior #7271, Replaces empty selected text blocks when inserting new blocks  #5054, Add Block (+) is inconsistently used, possibly confusing #8363
• Alt+F10 should navigate into the block toolbar regardless whether it is currently visible.
  • Tracked at Blocks: Alt+F10 should navigate to block toolbar regardless of visibility #10907

Testing instructions:

Verify that, after typing in a paragraph block, pressing Escape causes the toolbar to be shown.

[youknowriad]

LGTM 👍 Code wise. Would be good to have a quick accessibility check.

[youknowriad]

I'm still not sure what label we should use "Accessibility" or "Needs Accessibility Feedback" 🤷‍♂️