Update layout classnames and specificity

[isabel_brison]

This ticket tracks the changes to src/wp-includes/block-supports/layout.php, src/wp-includes/class-wp-theme-json.php and src/wp-includes/theme.json from the following Gutenberg PRs:

​https://github.com/WordPress/gutenberg/pull/47858
​https://github.com/WordPress/gutenberg/pull/47952

Because they touch the same parts of the same files and are conceptually close I think it makes sense to bring them in together.

Trac ticket: https://core.trac.wordpress.org/ticket/58548

This PR backports the PHP changes from ​https://github.com/WordPress/gutenberg/pull/47858 and
​https://github.com/WordPress/gutenberg/pull/47952.

To test the changes, build PR branch locally. and make sure a block theme such as Twenty Twenty Three is enabled. Until the npm packages are updated, these changes will only be testable on the front end.

• The compound classname should appear on all blocks with layout enabled, e.g. in TT3, the main element in the home template should have a wp-block-group-is-layout-constrained class, in addition to the previously existing classes.
• Child blocks of flow and constrained layouts (such as the children of the above main element) should have the reduced specificity rules applied to them.

[poena]

Hi
I am sorry if this is a stupid question or remark, but in this PR, we are updating

theme.json > layout definitions > spacingStyles,

and in https://core.trac.wordpress.org/ticket/58550 we are moving the layout definitions to block-supports/layout.php:
​https://github.com/WordPress/wordpress-develop/pull/4624/files#diff-859f1673f11bdb7ef6b2b9d1ab516b06ce464f8d065d129beacd94cd23888cc0R22

[isabel_brison]

Yes, in order to make these changesets easier to review, Andrew and I decided to add them incrementally. This change is older than the one in #58550; the plan is to get it committed first and then add #58550 on top.

[isabel_brison]

In 55956:

Editor: update layout classnames and specificity.

Adds a compound layout classname and reduces layout spacing rule specificity.

Props ramonopoly, andrewserong, poena.
Fixes #58548.

Thanks for the reviews, folks!

Committed in r55956 / ​cfe8ab9.