feat: add reusable announceToScreenReader helper and refactor block.js (#6608)

[abhnish]

Description

Extracts the aria-live region creation logic into a reusable announceToScreenReader helper function in js/utils/utils.js. Refactors the drag announcement in js/block.js to use this new helper instead of an inline IIFE.

This centralizes screen reader announcements, avoiding duplicate DOM creation logic and making it easier to trigger silent assistive announcements across the rest of the codebase.

Related Issue

Part of #6608

Category

• Feature
• Refactoring

Type of Change

• Accessibility Enhancement
• Code Quality

Testing Performed

1. Opened the app locally at localhost:3000, enabled VoiceOver on macOS.
2. Dragged a block — "picked up [block name]" was announced by screen reader.
3. Confirmed no visual message appears — aria-live region only.
4. Confirmed mbA11yLiveRegion is correctly lazy-loaded into the DOM on the first call and reused for subsequent announcements.
5. npx prettier --check js/block.js js/utils/utils.js — clean.

PR Checklist

• I have tested these changes locally
• My changes generate no new warnings or console errors
• I have checked for and resolved any merge conflicts

[abhnish]

@walterbender , the code for the aria-live helper and block.js refactor is ready. Note: The CI is currently failing on pitchstaircase.test.js and musickeyboard.test.js, but I verified this PR does not touch those files. It appears those tests are currently failing on master.

[walterbender]

Are there any other places we should be using this utility?

[abhnish]

@walterbender Yep, A few spots still need it:
activity.js: ensureA11yLiveRegion and the textMsg/errorMsg hooks (from PR #7674) are duplicating this pattern, blocks.js: the trash announcement in PR #7753 (still pending) still has that old inline IIFE.
Going forward, any new announcements (like widgets opening/closing or blocks snapping) will just use this helper directly.

will clean up activity.js and update blocks.js in a quick follow-up once #7753 merges!