submenu style

[mdawaffe]

FF screengrab attached.

(This might be a dupe - couldn't find it, though)

[Sewar]

Duplicate of #3149, and it has a patch.

[matt]

I think this is addressed in #3149.

❗ BLOCKED by #3218 (that PR will output the layout styles generated by this PR)

This PR is a follow-up to ​https://github.com/WordPress/wordpress-develop/pull/3205 and backports the remaining parts of the layout block support.

## To-do

• Backport tests
• Ensure it's working on top of #3218

Trac ticket:

@tellthemachines I believe this one is working correctly — it just depends on ​https://github.com/WordPress/wordpress-develop/pull/3218 in order for the generated styles to be output.

This is another fork of #3218, to attempt to fix the tests that are broken in #3259.

Original PR desc:

Backporting changes to script-loader that enqueue block support styles stored by the style engine/

See tracking issue: ​https://github.com/WordPress/gutenberg/issues/43440

Backporting the following changes

• ​https://github.com/WordPress/gutenberg/pull/42880

Trac ticket:

Thanks for all the feedback @costdev! I believe I've implemented it all, so this should be ready for another look now 🙂

Alrighty, now that ​https://github.com/WordPress/wordpress-develop/pull/3273 has landed, it looks like this one should be good to commit now CC: @ockham

Closing now that ​https://github.com/WordPress/wordpress-develop/pull/3273 has landed instead.

I've given this a rebase, and confirmed that it's still working correctly on top of trunk 👍

I'm self-assigning for review and commit.

## Test Report

### Steps to Test

1. Apply the PR.
2. Activate the Twenty Twenty-Two theme. Ensure that there are no header customizations in the Site Editor.
3. Navigate to the frontend.
4. Inspect the header row.

### Expected Results

• ✅ The element should now have the .is-layout-flex class.
• ✅ The styling should match that shown in the screenshot in the PR's description.

### Environment

• Server: Apache (Linux)
• WordPress: 6.1-alpha-53344-src
• Browser: Chrome 105.0.0.0
• OS: Windows 10
• Theme: Twenty Twenty-Two
• Plugins: None activated.

### Actual Results

• ✅ The element has the .is-layout-flex class.
• ✅ The styling matches that shown in the screenshot in the PR description.

### Supplemental Artifacts

For this backport, I'll be reverting the DocBlock changes for wp_get_layout_style(). Why?

• They were not part of the backports from Gutenberg's PR 40875 ​https://github.com/WordPress/gutenberg/pull/40875.
• And they were introduced in 5.9.

Instead, I'll do a follow-up coding standards commit to improve the DocBlock including updating the descriptions for the optional parameters (e.g. adding the word Optional. and adding the default Default is null.).

Committed via changeset https://core.trac.wordpress.org/changeset/54274. Thank you everyone for your contributions!

A follow-up commit is coming to address other changes that were not part of the original GB PR (such as DocBlock improvements).

DocBlock improvements suggestioned in this PR were committed via changeset https://core.trac.wordpress.org/changeset/54275.