fix: Toolbox & Flyout ARIA positions (experimental)

[BenHenning]

The basics

• I validated my changes

The details

Resolves

Fixes #9386
Fixes part of #9293

Proposed Changes

Addresses #9386 through a number of changes:

• Ensures the flyout contents are reevaluated for ARIA changes whenever they themselves change (since previously WorkspaceSvg only recomputed its ARIA properties when one of its blocks self-registered which doesn't account for labels or buttons).
• Collapsible categories are now correctly wrapped by a group (since groups of tree items must be in a parent group).
• Updates toolbox ARIA computations to be on content changes rather than having items self-specify recomputing the ARIA properties. This mitigates an issue with collapsible categories not updating the toolbox contents and thus being omitted.
• Introduced a separate pathway for computing tree info for flyout workspaces (vs. for the main workspace) in order to account for FlyoutButtons.
• Updated FlyoutButton to use a nested structure of treeitem then button in order to actually count correctly in the tree and still be an interactive button. The readout experience is actually better now on ChromeVox, as well, since it reads out both 'Button' and 'Tree item' which is interesting. It seems screen readers are designed to look for this pattern but it only works if set up in a very particular way.

Reason for Changes

Most of the changes here fixed incidental problems noticed while trying to address #9386 (such as the variables category not correctly accounting for the 'Create variable' button in the count, or not having the correct labels). Much of the count issues in the original issue were caused by a combination of missing some flyout items, and trying to compute the labels too early (i.e. before the categories were fully populated).

Test Coverage

Since this is an experimental change, no new tests were added.

Documentation

No documentation changes are directly needed here.

Additional Information

None.

[BenHenning]

Self-review.

[BenHenning]

Self-review latest.

[BenHenning]

I don't think the latest change needs another review pass since it's just adding a TODO and cleaning up redundant isFocusableNode calls in WorkspaceSvg. Will go ahead and merge this once CI is done running.