Plugin: Extract block editor styles replacement as filter

[aduth]

Extracted from: #13569
Related: #9008

This pull request seeks to implement Gutenberg's replacement editor-styles setting value as a filter on block_settings. This was allow for compatibility with the core implementation once Gutenberg stops reimplementing its own replacement editor (#13569).

Implementation notes:

The change to assign $styles with ABSPATH . WPINC . '/css/dist/editor/editor-styles.css' is merely meant to provide direct alignment to the core implementation. It's never actually used, since the filtering behavior will substitute the styles with its own implementation. But it needs to exist because the filtering assumes that the styles would be the first entry of the default $settings['styles'] array. Unfortunately, we must operate on assumptions since the entries of styles are the contents of the stylesheets themselves, not named or assigned as paths. An alternative implementation could consider reading from the ABSPATH . WPINC to find and substitute the value by the actual file contents assigned, but there'd be a small amount of overhead in reading from the filesystem.

Testing instructions:

Verify that the contents of the built copy of packages/editor/src/editor-styles are present on the page. This may require inspecting the source of the page. It's easier to verify that it's in-fact the Gutenberg copy by making a change to the source file and rebuilding it.

Additionally, verify that the separate locale font family assignment (font-family: 'Noto Serif') is not impacted by these changes, meaning that it is still present and occurs after the editor-styles.css.

[youknowriad]

I wonder if it's simpler to just loop over the styles are replace the core editor styles if found (checking the path or something).

[aduth]

replace the core editor styles if found (checking the path or something).

The problem is that the array contains only the CSS string, no file paths.

It looks something like:

$settings = array(
	'styles' => array(
		array( 'css' => 'body { /* ... */ }' )
	)
);

So we've got nothing to work from to know whether it's the core style, unless we explicitly (re-)read the ABSPATH . WPINC . '/css/dist/editor/editor-styles.css' file from disk and compare it with the array style contents.

[aduth]

Let's just do that.

[aduth]

Default styles matching made more reliable in d4be401 by exact file contents match.

Fun testing instructions:

echo "* { font-family: \"Comic Sans MS\", cursive, sans-serif \!important; }" >> build/editor/editor-styles.css