fix: Clean up accessibility node hierarchy (experimental)

[BenHenning]

The basics

• I validated my changes

The details

Resolves

Fixes #9304

Proposed Changes

Adds relevant aria-owns properties to ensure that all focusable accessibility nodes have the correct parent/child relationships (particularly for blocks).

Reason for Changes

Other PRs have made progress on removing extraneous accessibility nodes with #9446 being essentially the last of these. Ensuring that parent/child relationships are correct is the last step in ensuring that the entirety of the accessibility node graph is correctly representing the DOM and navigational structure of Blockly.

This can have implications and (ideally) improvements for certain screen reader modes that provide higher-level summarization and sometimes navigation (bypassing Blockly's keyboard navigation) since it avoids an incorrect flat node structure and instead ensures correct hierarchy and ordering.

Test Coverage

No automated tests are needed for this since it's experimental. However, manual testing involved adding various types of blocks and fields plus icons and mutators to ensure that relevant elements were correctly owned.

Documentation

No documentation changes are needed for these experimental changes.

Additional Information

Note that there are some limitations with this approach: text editors and listboxes (e.g. for comboboxes) are generally outside of the hierarchy represented by the Blockly workspace. This is an existing issue that remains unaffected by these changes, and fixing it to be both ARIA compliant and consistent with the DOM may not be possible (though it doesn't seem like there's a strong requirement to maintain DOM and accessibility node tree hierarchical relationships).

This is likely the cause of certain "region end"-esque announcements and may require additional investigation and work. It's considered outside the scope of this PR and #9304.

Finally, the changes here are rather ugly and would benefit from some more careful long-term technical design considerations (a la #9307).

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[BenHenning]

Looks like there's some CI failures to address. Moreover I haven't completed in-depth testing of the hierarchy--I want to check more block and fields types before considering this ready for review.