Blocks: Fix relative URLs for inlined styles

[aristath]

Description

Issue was reported in https://core.trac.wordpress.org/ticket/54243 and a patch was pushed in WordPress/wordpress-develop#1752

This PR backports the same mods to Gutenberg.

When a stylesheets gets inlined, any relative URLs in that stylesheet break.
The problem is that URLs inside CSS files are relative to the stylesheet's path, and when styles get inlined that relation is lost. This PR fixes the issue by finding relative URLs which then get modified to be relative to the site's root.

How has this been tested?

Tested by adding the following CSS in a block stylesheet, and then making sure that URLs get properly updated.
Note: The CSS below is wrong and meaningless, but it does test the issue efficiently.

.foo {
  background:url( "image0.svg" );
  background-image: url('./image2.png');
  background-image: url('../image1.jpg');
  background-image: url('http://foo.com/image2.png');
  background-image: url( 'https://bar.com/image2.png');
}

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[spacedmonkey]

@aristath Can you add a PHP unit test to this? Any code, specially with regex in it, that is not readable, 100% needs a test to prove that is works.

[aristath]

Tests were added in the patch for core, so I don't think we need to copy these tests to Gutenberg, do we?

[gziolo]

@aristath, can you refresh this patch? I think that e2e test failures are completely unrelated. I tested the core patch today: https://core.trac.wordpress.org/ticket/54243. I would be happy to land this change as well. Ideally, before the 11.9 RC1 planned for tomorrow.