Theme JSON: adding optional argument to get_style_nodes()

[ramonjd]

What?

Adding an optional, second $selectors argument to get_style_nodes() to match the original function signature in 5.9.

Props to @torounit, who discovered the bug and came up with the original fix in #41217

This PR includes an additional type check in filter_out_block_nodes() to avoid the PHPCodeSniffer error:

Fatal error: Uncaught TypeError: vsprintf(): Argument #2 ($values) must be of type array, string given in /app/vendor/squizlabs/php_codesniffer/src/Files/File.php:1056

Why?

#41160 refactored get_style_nodes() so that the $selectors argument was no longer required. This was incompatible with the original method in 5.9.

How?

Adding an optional $selectors argument to keep the linters happy.

Testing Instructions

Fire up this branch and run through the testing scenarios in #41160

Ensure you don't encounter the error:

Warning: Declaration of WP_Theme_JSON_6_1::get_style_nodes($theme_json) should be compatible with WP_Theme_JSON_5_9::get_style_nodes($theme_json, $selectors = Array) in /var/www/html/wp-content/plugins/gutenberg/lib/compat/wordpress-6.1/class-wp-theme-json-6-1.php on line 124

[ramonjd]

Closing in favour of original PR
#41217