aria live region for announcements

[maribethb]

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Problem

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Request

Implement the aria live region for announcing updates on Blockly. This can be partially backported out of the screen-reader branch, with some improved API niceties.

You can find the details for the implementation here, in section 4.2.1

Some highlights, please refer to the doc for more details:

• The region must be displayed, as hidden elements are ignored by screenreaders, so you need a bit of hacky css to make it only "visible" to screenreaders.
• We should default to using "polite" mode for the aria-live region. Though reading the spec initially had us using assertive mode, we found in testing that some platforms wouldn't read assertive announcements at all. See [Screen reader] Review of aria-live region announcements #9460 for more details.
• We must use a debounce for updating the region to ensure announcements are always read. There is more discussion of this in [Screen reader] Review of aria-live region announcements #9460. If the debounce isn't already implemented in the screen-reader branch, microbit should have a PR somewhere with their implementation details we can crib from. I can dig this up if needed.
• The documentation for the methods should include clear guidelines about when to use it
• The region will be a page global created during injection using the aria utility API as described in the linked doc above.

Alternatives considered

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Additional context

We should probably close #9460 when this is closed as well, but I wanted to make a separate bug to make it more clear when this is merged into the v13 branch.

[maribethb]

@mikeharv This one has to be done before the i shortcut one. And when it is done you can also tackle the announcement side of #9625