Theme JSON: Define preset CSS vars for blocks based on feature selectors

[aaronrobertshaw]

Trac ticket: https://core.trac.wordpress.org/ticket/64598

This PR brings the changes from the following Gutenberg PR to core:

WordPress/gutenberg#75226

Description

Updates the global styles engine and theme JSON processing to generate preset CSS custom properties (vars) for blocks based on their feature-specific selectors rather than only the block's root selector. This enables blocks whose root selector targets an inner element to have preset CSS vars output on the appropriate outer wrapper selector.

See: WordPress/gutenberg#74242

Testing

1. Run the PHP unit tests: phpunit --filter=WP_Theme_JSON_Test
2. Verify all existing tests continue to pass, including the new test_get_stylesheet_preset_css_vars_use_feature_selector test.
3. For manual testing, register a block that defines a feature-level selector (e.g. a dimensions.root selector different from the block's root selector), define dimensionSizes presets for that block in theme.json, and confirm the CSS vars are output on the feature selector rather than the block's root selector.

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Props aaronrobertshaw, westonruter.

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[aaronrobertshaw]

The test failure is related. This will need to be rebased after #10628 lands.

Alternatively, I might be able to update the test to use an older preset in theme json, e.g. aspect ratio or something.

If I go that route I'll update the Gutenberg PR to match

[westonruter]

Array values, string|array, can array be made more specific? If not then it should probably be just:

Suggested change

	* @param array<string, string|array> $feature_selectors The block's feature selectors map.
	* @param string $feature_key The feature to look up (e.g. 'dimensions').
	* @param string $default_selector Fallback selector.
	* @param array<string, mixed> $feature_selectors The block's feature selectors map.
	* @param string $feature_key The feature to look up (e.g. 'dimensions').
	* @param string $default_selector Fallback selector.

[aaronrobertshaw]

The feature selectors could be an array of string key/value pairs, where the value could be a string or a further array of string key value pairs for block support sub features. The block.json schema outlines it here but it would looks something like:

{
  "root": string,
  "border": string | array<string, string>,
  "color": string | array<string, string>,
  "dimensions": string | array<string, string>,
  "spacing": string | array<string, string>,
  "typography": string | array<string, string>
}

I will update this to array<string, string|array<string, string>> for complete accuracy. If it is too verbose we can switch it up to the array<string, mixed> if desired.

[westonruter]

Is it possible that $feature['root'] could possibly not be a string? If not, this would be safer as:

Suggested change

	return $feature['root'] ?? $default_selector;
	if ( isset( $feature['root'] ) && is_string( $feature['root'] ) ) {
	return $feature['root'];
	}
	return $default_selector;