Allow assigning CSS variables in safecss_filter_attr

[aristath]

The safecss_filter_attr allows using custom CSS variables like color: var(--color);. However, it does not currently allow assigning values to CSS variables (like --color: #F00) - which is something we do a lot in global-styles and Gutenberg.
We should allow assigning values to CSS variables so we can use this function consistently in Global Styles and the future Style Engine (WIP in Gutenberg)

Trac ticket: https://core.trac.wordpress.org/ticket/56353#ticket

A bit late. ​https://github.com/WordPress/wordpress-develop/pull/3078 illustrates the solution.

I'm just putting this up there to see the tests pass 😄

## Proposal

​CSS custom properties don't pass the tests in safecss_filter_attr() at ​if ( in_array( $css_selector, $allowed_attr, true ) ) because there's no allowing entry in the allowed_attrs array.

This is a PR to stimulate discussion around whether, and how, we WordPress could be permissive of such properties.

Trac ticket: https://core.trac.wordpress.org/ticket/56353

[ramonopoly]

Thanks for getting this ticket open @aristath. I think it's a great opportunity to open the discussion and document the pros and cons.

Do any folks have any links to previous threads that cover this issue?

All I could find was #46498, but there's no patch attached.

Closing in favour of ​https://github.com/WordPress/wordpress-develop/pull/3078

[aristath]

Pushed an extra tweak to the patch and now tests pass

[SergeyBiryukov]

In 54117:

KSES: Allow assigning values to CSS variables.

The safecss_filter_attr() function allows using custom CSS variables like color: var(--color). However, it did not allow assigning values to CSS variables like --color: #F00, which is common in Global Styles and Gutenberg.

This commit adds support for assigning values to CSS variables, so that the function can be used consistently in Global Styles and the future Style Engine in Gutenberg.

Follow-up to [50923], [54100].

Props aristath, ramonopoly, SergeyBiryukov.
Fixes #56353.

Thanks for the PR! Merged in r54117.