[Do not merge] Test a11y plugin changes v2

[BenHenning]

This is a working branch for bringing in FocusManager support in the keyboard navigation plugin.

Other things left to do:

• Rendered connections need to be redone so that they correctly update due to block moves and resizes.
• The cursor may need to be reimplemented further such that its state is entirely derived from FocusManager rather than partially.
• Clicking needs to tie back to focus or the state becomes inconsistent. This has been partly implemented, but is not yet complete.
• Styling choices need to be finalized. CSS works, but SVG rendering issues cause strokes to be partially cut off. We will need to introduce an extra path element per block to represent the block's highlights (for selection or passive highlight).

Pathways untested (or partially tested):

• Comments and icons, and more complex flyout arrangements.
• Action menu.
• Keyboard-based movement.
• Other shortcuts.
• Likely other scenarios that were carefully patched over the last few months that now fundamentally change with this approach.

Cool thing to note: this works with Chromevox, though it's rough since group elements over-announce and we don't have good block representations yet. Much of this will be fixed with ARIA work, though. :)

The corresponding core change is here: RaspberryPiFoundation/blockly#8875. Note that the core branch and this branch must be linked together, e.g. using https://developers.google.com/blockly/guides/contribute/samples/debugging#npm_link in order for both to work correctly.