Set up experimental branches for screen reader development

[BenHenning]

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Problem

Introducing ARIA support (#8207) and broader screen reader work (#3370) will be done in an experimental way in order to enable quick iteration on readout details (RaspberryPiFoundation/blockly-keyboard-experimentation#673).

Request

Multiple experimentation branches should be set up with branch protection rules and cross-CI linking between core Blockly and the experimentation plugin.

Alternatives considered

No response

Additional context

No response

[BenHenning]

Note: add-screen-reader-support-experimental has been created for both core and the plugin repositories and has been given branch protection statuses to ensure only PRs can change it (and that it cannot be deleted).

The only other part left to wrap up this issue is to get CI working correctly across both repositories (since the two branches need to link to each other)--this will happen after #9280 is merged.