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, ramonopoly, westonruter, wildworks.

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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

[t-hamano]

I would appreciate it if you could proceed with this PR and commit it in time for the 7.1 Beta1 release on July 15.

[ramonjd]

@aaronrobertshaw This was prepped before 7.0 and probably punted by default.

I've rebased and prepped things, but would like to know if it's okay to commit.