DropdownMenu V2: add fallback styles for when subgrid is not supported

[ciampo]

What?

Add fallback styles to the experimental DropdownMenu v2 component for browsers that don't support CSS subgrid

Why?

As of today, there are still a few browsers with > 1% usage that don't support CSS subgrid (old safari versions, opera mobile, Samsung internet) that we should still support

How?

Add some fallback styles so that the menu items don't look broken in browsers that don't support CSS subgrid. The main difference is that the auto-indentation is not supported in those browsers.

Note: these are fallback styles that only a very small (and decreasing every day) percentage of folks are going to experience. The goal is to provide a non-broken experience, even if the overall look is not as refined as in modern browsers

Testing Instructions

To test what it's like to use the DropdownMenu component in an environment that doesn't support CSS subgrid:

• use an older version of a web browser locally (ie. install Chrome 116), or;
• use an older version of a web browser via a cloud service (ie. Browserstack), or;
• manually delete from this PR the code inside @supports ( grid-template-columns: subgrid ) query

Test the DropdownMenu V2 Ariakit component both in Storybook and in the editor, on both a browser that doesn't support CSS subgrid and on one that supports it.

• Make sure that the fallback layout works well enough
• Make sure that the "standard" layout looks exactly like on trunk

Screenshots or screencast

In a browser with no CSS subgrid support (trunk)	In a browser with no CSS subgrid support (This PR)

[tyxla]

Works well and looks great! 🚀

[github-actions]

Flaky tests detected in 33b5f06.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7298991546
📝 Reported issues:

• [Flaky Test] Should show Columns block only if selected blocks are between limits (1-6) #40369 in specs/editor/various/block-switcher.test.js