Editor: Fix bumped specificity for layout styles in non-iframed editor.

Fixes a regression introduced in [58241] which inadvertently bumped the specificity in a non-iframed editor for `.editor-styles-wrapper .is-layout-flow > *` from (0,1,0) to (0,2,0). This fix restores theme.json spacing rules taking precedence over the implicit spacing rules in a non-iframed editor.

**The What**

When the block editor is not iframed (which can happen when Custom Fields are active, or blocks that use and older `apiVersion` are present), style rules are processed using post css to append the `.editor-styles-wrapper` class name. This has the effect of scoping the the style rules to ensure they don't affect the editor chrome or admin.

With [58241], one of the rules was changed to `.is-layout-flow > *`. In a iframed editor, the specificity of this rule is okay (0,1,0), but in a non-iframed editor it becomes `.editor-styles-wrapper .is-layout-flow > *`, a specificity of (0,2,0). Comparing this to before [58241], the same rule was `.editor-styles-wrapper :where(body .is-layout-flow) > *` (specificity 0,1,0). This is a regression in specificity that has caused some issues. Notably themes can no longer properly override the spacing for blocks using theme.json and have the results correctly shown in the non-iframed editor.

**The How**

This changeset modifies the selector to `:root :where(.is-layout-flow) > *` (still specificity 0,1,0). `transformStyles` handles 'root' selectors a little differently, it'll instead replace the `:root` part so it becomes `.editor-styles-wrapper where(.is-layout-flow) > *` (keeping the specificity at 0,1,0).

The other layout selector that this affects is the `:first-child` `:last-child` selectors that are responsible for resetting margin at the start and end of a block list. They traditionally have a 0,2,0 specificity so that they can override both the above rule and any rules in the theme.json. Those selectors are also maintained at 0,2,0 with this change, they become something like `:root :where(.is-layout-flow) > :first-child`.

**References:**
* PHP changes from [WordPress/gutenberg#64076 Gutenberg PR 64076].

Reviewed by andrewserong.
Merges [58890] to the 6.6 branch.

Follow-up to [58241], [58228], [55956], [54162].

Props talldanwp, aaronrobertshaw, andrewserong, markhowellsmead, ramonopoly, hellofromTonya, munyagu, apmeyer.
Fixes #61829.

git-svn-id: https://develop.svn.wordpress.org/branches/6.6@58986 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -1681,7 +1681,7 @@ protected function get_layout_styles( $block_metadata, $types = array() ) {
 										$spacing_rule['selector']
 									);
 								} else {
-									$format          = static::ROOT_BLOCK_SELECTOR === $selector ? '.%2$s %3$s' : '%1$s-%2$s %3$s';
+									$format          = static::ROOT_BLOCK_SELECTOR === $selector ? ':root :where(.%2$s)%3$s' : ':root :where(%1$s-%2$s)%3$s';
 									$layout_selector = sprintf(
 										$format,
 										$selector,