Editor: Backport foundation for Layout block support refactor (part 1).

Backports the following changes from the Gutenberg repository:

* [WordPress/gutenberg/40875 gutenberg/40875] Layout: Use semantic classnames, centralize layout definitions, reduce duplication, and fix blockGap in theme.json
* [WordPress/gutenberg/42544 gutenberg/42544] Layout: Add a disable-layout-styles theme supports flag to opt out of all layout styles gutenberg/42544
* [WordPress/gutenberg/42087 gutenberg/42087] Theme.json: Add block support feature level selectors for blocks gutenberg/42087
* [WordPress/gutenberg/43792 gutenberg/43792] Global Styles: Split root layout rules into a different function gutenberg/43792
* [WordPress/gutenberg/42544 gutenberg/42544] Layout: Add a disable-layout-styles theme supports flag to opt out of all layout styles gutenberg/42544
* [WordPress/gutenberg/42665 gutenberg/42665] Layout: Reduce specificity of fallback blockGap styles gutenberg/42665
* [WordPress/gutenberg/42085 gutenberg/42085] Core CSS support for root padding and alignfull blocks gutenberg/42085

Notes:
* It doesn't entirely port over PR 40875 — the remaining PHP changes for that PR will be explored in a separate PR targeting `layout.php`.
* [54159] was reverted in [54160] due to PHPUnit test failures for tests added by the commit. Later, tests passed when applied on top of `trunk`. There were various outages today of upstream `wp-env` dependencies, which likely were the root cause of the earlier failures. For historical tracking and to make sure, recommitting [54159] but instead on top of current `trunk`. See PR 3205 for more details.
* Giving additional props for those who did a deep dive investigation into the failed tests.

Follow-up to [54160], [54159].

Props andrewserong, aaronrobertshaw, isabel_brison, bernhard-reiter, hellofromTonya.
See #56467.
Built from https://develop.svn.wordpress.org/trunk@54162


git-svn-id: http://core.svn.wordpress.org/trunk@53721 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -4026,6 +4026,13 @@ function create_initial_theme_features() {
 			'show_in_rest' => true,
 		)
 	);
+	register_theme_feature(
+		'disable-layout-styles',
+		array(
+			'description'  => __( 'Whether the theme disables generated layout styles.' ),
+			'show_in_rest' => true,
+		)
+	);
 	register_theme_feature(
 		'editor-color-palette',
 		array(