Description

Fixes #31352.

This PR does a number of things:

1. It brings back the Placeholder component so we can style according to container widths
2. It tweaks the placeholder skeleton indicators to match the height of actual menu items
3. It lets skeleton indicators wrap.
4. It adjusts the visual appearance of the setup state to be stacked in constrained contexts.
5. It rephases the verbiage of the button when you have existing menus.

1 was the key bit missing. The Placeholder component comes with a "resizeobserver" and assigns is-small, is-medium and is-large classes based on the width of the block. This allows us to stack the setup state in very small contexts.

2 and 3 are side-effects of general refactor efforts to remove an "on-select jump", which is better described in #31198 which this PR also replaces. While this onselect thing didn't need to be fixed in this PR, it touched the same code anyway, and made sense to me to include since I was refactoring for various container widths.

4 was the key one. The setup state could be very broken in constrained contexts, for example inside a columns block. This PR fixes that:

5 rephases the "Existing menu" button to instead say "Add existing menu":

Existing menus are created in the separate Navigation screen (both the legacy and the new block-based one). The clarification can hopefully help suggest that the existing menu is meant as a starting point, because there isn't actually a two-way connection between the two. In other words, if you add an existing menu to your navigation block, and later add new menu items on the menu screen, those will not show up in the Navigation block.

How has this been tested?

Please test the navigation block in its setup state. Both the horizontal and vertical versions:

• Verify it looks correct just as-is.
• Verify it looks good when inserted inside a colums block or other narrow context.
• Test inserting any block inside the navigation.
• Test with "Start empty", and verify the skeleton indicators look good.

I'm still thinking about whether the skeleton indicators are a good idea at all, and that's something to ponder. But for now they work reasonably well, and most of all they share metrics of the theme now.

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).