Global Styles: fix presets that use a callback to validate user data

[oandregal]

Follow-up to #34667
Related #35228 #35248

This PR makes the remove_insecure_settings take the value from the callback registered by the preset via value_func, if it exists.

Note that for allowing the user to create their own duotones via the global styles sidebar, we'd need to also update or hook into the safecss_filter_attr function from WordPress core. I'm not doing this here because we don't need it so far.

How to test

• Paste the following function at the end of lib/class-wp-theme-json-gutenberg.php (alternative create your own function in any file that's in scope):

/**
 * Test function.
 *
 * @param array $preset Color preset.
 *
 * @return string Color value.
 */
function return_color_key_value_from_preset( $preset ) {
	return $preset['color'];
}

• In PRESETS_METADATA, remove the value_key of the first preset (color.palette) and paste this instead: 'value_func' => 'return_color_key_value_from_preset',. This is telling the preset will look up for the value via the callback we created.

Then, run npm run test-php: the expected result is that all the tests still pass. Also verify that the tests will fail in trunk if we use value_func for the color.palette.