Description

When inserting or selecting a block with a placeholder, VoiceOver doesn't read out the description, so there's often not enough context to know what the controls are supposed to do.

This PR changes the placeholder markup slightly to wrap the contents in a fieldset, and the description in a legend. This causes the description to be read by VoiceOver when the first control is focused.

Testing Instructions

1. Add a Columns or Table block to a post.
2. Move focus into the block placeholder by pressing Arrow Down (this won't work if "Contain text cursor inside block" is enabled, see Fix useFocusFirstElement in non-contenteditable Blocks #37934)
3. Verify that VoiceOver reads the placeholder description.

I haven't checked with any Windows screen readers, but previous testing shows this isn't an issue on NVDA, so it would be good to double-check this doesn't make the experience worse for Windows users.

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.