The basics

• I validated my changes

The details

Resolves

Fixes #9166 and part of RaspberryPiFoundation/blockly-keyboard-experimentation#636.

Proposed Changes

This PR adds support for keyboard navigation to/from block comments. To that end:

• TextInputBubble is refactored to reuse CommentEditor (also used by workspace comments) to handle text editing
• Bubbles have been updated to close themselves when Escape is pressed
• Several additional navigation policies have been added.

A corresponding change will be needed in keyboard-experiment, and tests will be added there.