feat: Add initial support for screen readers (experimental)

[BenHenning]

The basics

• I validated my changes

The details

Resolves

Fixes part of #8207
Fixes part of #3370

Proposed Changes

This introduces initial broad ARIA integration in order to enable at least basic screen reader support when using keyboard navigation.

Largely this involves introducing ARIA roles and labels in a bunch of places, sometimes done in a way to override normal built-in behaviors of the accessibility node tree in order to get a richer first-class output for Blockly (such as for blocks and workspaces).

Reason for Changes

ARIA is the fundamental basis for configuring how focusable nodes in Blockly are represented to the user when using a screen reader. As such, all focusable nodes requires labels and roles in order to correctly communicate their contexts.

The specific approach taken in this PR is to simply add labels and roles to all nodes where obvious with some extra work done for WorkspaceSvg and BlockSvg in order to represent blocks as a tree (since that seems to be the best fitting ARIA role per those available: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Roles). The custom work specifically for blocks includes:

• Overriding the role description to be 'block' rather than 'tree item' (which is the default).
• Overriding the position, level, and number of sibling counts since those are normally determined based on the DOM tree and blocks are not laid out in the tree the same way they are visually or logically (so these computations were incorrect). This is also the reason for a bunch of extra computation logic being introduced.

One note on some of the labels being nonsensical (e.g. 'DoNotOverride?'): this was done intentionally to try and ensure all focusable nodes (that can be focused) have labels, even when the specifics of what that label should be aren't yet clear. More components had these temporary labels until testing revealed how exactly they would behave from a screen reader perspective (at which point their roles and labels were updated as needed). The temporary labels act as an indicator when navigating through the UI, and some of the nodes can't easily be reached (for reasons) and thus may never actually need a label. More work is needed in understanding both what components need labels and what those labels should be, but that will be done beyond this PR.

Test Coverage

No tests are added to this as it's experimental and not a final implementation.

The keyboard navigation tests are failing due to a visibility expansion of connectionCandidate in BlockDragStrategy. There's no way to avoid this breakage, unfortunately. Instead, this PR will be merged and then google/blockly-keyboard-experimentation#684 will be finalized and merged to fix it. There's some additional work that will happen both in that branch and in a later PR in core Blockly to integrate the two experimentation branches as part of #9283 so that CI passes correctly for both branches.

Documentation

No documentation is needed at this time.

Additional Information

This work is experimental and is meant to serve two purposes:

• Provide a foundation for testing and iterating the core screen reader experience in Blockly.
• Provide a reference point for designing a long-term solution that accounts for all requirements collected during user testing.

This code should never be merged into develop as it stands. Instead, it will be redesigned with maintainability, testing, and correctness in mind at a future date (see google/blockly-keyboard-experimentation#673).

[BenHenning]

There might need to be a bit of branch-fu in order to make the tests work correctly between this new long-lived core branch and the equivalent version in the keyboard navigation repo.

[BenHenning]

Next steps here:

• File an issue to track this work. (Edit: Using Use ARIA in core Blockly #8207 since it already exists)
• Audit the remaining TODOs that were removed from feat: Introduce initial screen reader support (experimental) blockly-keyboard-experimentation#684 to make sure nothing was missed.
• Audit against the original experimentation PR to ensure other core-specific things can be re-added (since it's easier without the monkey patching), per feat!: Temp branch for screen reader concepts [DO NOT MERGE] #9262.
• Figure out a plan for getting cross-repo CI working.
• Finalize names for the base branches (maybe we should add '-experimental'?).
• Get CI passing on this PR and get it into review.

[microbit-matt-hillsdon]

The aria bit needs to be after doInit_() for the block to have fields.

[BenHenning]

Nice catch. The dataflow of when things happen has changed each time the code was moved, so I missed this and somehow missed the voice output being incomplete for blocks.