Injected editor-styles invalidate :root declarations thus breaking custom properties

[mor10]

Describe the bug
Same issue, but different reasoning than #11955, which was closed without resolution in November 2018. The issue deserves another look and a solution as the :root pseudo-class is a core element of CSS custom properties which will see significantly increased use as browsers now provide wide-spread support:

When a theme uses CSS custom properties (aka "CSS variables") and declare those properties using the :root pseudo-class in the editor styles, the injection script invalidates those declarations by outputting .editor-styles-wrapper :root.

To Reproduce
Steps to reproduce the behavior:

1. In editor styles document, declare CSS custom properties with :root pseudo-class.
2. Use above custom properties in another rule.
3. Add editor style using add_editor_style().
4. Run in browser.
5. Observe the declared custom properties not kicking in.
6. Inspect code and find the :root pseudo-class has the .editor-styles-wrapper class prepended.

Expected behavior
:root pseudo-class should be preserved without alterations.

Desktop (please complete the following information):

• OS: All
• Browser: All with custom property support
• Version: trunk

Additional context
See WP Rig PR #332 and issue #361 for further discussion and original discovery of the issue by @benoitchantre.

[mor10]

Filed as Trac ticket as well: https://core.trac.wordpress.org/ticket/46435#ticket

[youknowriad]

This was fixed in #13325 (will land in WP 5.2)