Block switcher rmenu accessibility and labeling improvements

[afercia]

Description

This came into my mind while providing a11y feedback on #60810

The block switcher opens a dropdown menu that is an ARIA menu pattern. ARIA menus can only contain children with role menuitem, menuitemcheckbox, menuitemradio and, optionally, groups with separators.

Visually, the menu is spit into two main groups. Semantically, there are two main groups as well. See attache dscreenshot to compare the visuals with the accessibility tree.

However, while the first group is labeled 'TRANSFORM TO`, the second group is unlabeled.

As a user, I'm not sure I would understand why there's a visual separation in two groups. What the difference between the two groups is?

Semantically, it doesn't make much sense labeling only the first group. Grouping items together makes only sense if the scope and purpose of the group is clear and meaningful. Is there a good reason to have two main groups in the first place? If there is one, than both groups should be visually and semantically labeled,

Note: the third group with the 'this block is connected' information should be removed as pointed out on #60810 (comment)

Step-by-step reproduction instructions

• Select a block.
• Open the block switched from the block toolbar.
• Note: if the block is connected to external sources via the Bindings API, it will show a third group at the bottom. This is not much relevant for this issue.
• Observe the menu is visually split in two groups;
• Observ the second group is not labeled.
• Optionally, inspect the accessibility tree in your browser dev tools and observe the secodn group is unlabeled.

Screenshots, screen recording, code snippet

No response

Environment info

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Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes