Update/editor styles wrap

[timelsass]

Description

Refer to #11955 - as this would resolve the issue by adding support for the editor-styles wrap for stylesheets using :root selector.

How has this been tested?

Have only done initial testing with the specificity to ensure that's correct and updated the tests with new snapshots currently.

Types of changes

Bug fix: Stylesheets making use for :root selector are now able to be handled.

Checklist:

• [ x ] My code is tested.
• [ x ] My code follows the WordPress code style.
• [ x ] My code follows the accessibility standards.
• [ x ] My code has proper inline documentation.

[timelsass]

For the replacement of :root it uses the namespace plus an invalid negation:
c749422#diff-29a319a31b11ad24eb78db1f40d51756R25

According to documentation for simple selectors [6], negation selectors [6.6.7] can allow for useless selectors to be written, but given a higher specificity. The specificity applied is how :root gains priority over html - which gives the wrap a basic implementation of .namespace:root in a way.

[gziolo]

Given that parent issue #11955 was closed and there wasn't clear agreement that this change is very expected, I'm inclined to close this PR. Thank you @timelsass for opening the original issue and sharing the proposed solution. If you think it's something we should still include in Gutenberg, feel free to re-open PR.