fix!: Improve keyboard navigation of icons and bubbles

[gonfunko]

The basics

• I validated my changes

The details

Resolves

Fixes #9720

Proposed Changes

This PR improves the interaction of keyboard navigation with icons and bubbles. Now, for all kinds of built-in bubbles:

• Open bubbles are treated as part of the first row of the block, and are navigable with left/right
• When a bubble is focused, M may be used to move it, Escape may be used to close it, and (for mutator and comment bubbles) Enter may be used to move to its mutator workspace/comment editor
• Mutator bubbles don't highlight the text of blocks in the mutator toolbox when they are focused

Breaking Changes

• Blockly.BlockCommentNavigationPolicy has been replaced with a more-generic Blockly.BubbleNavigationPolicy, which should be used in place of it.