Add kses support for complex CSS grid values including `repeat`, `auto-fill` and `auto-fit`

[isabel_brison]

In order for the new grid layout type from Gutenberg ​https://github.com/WordPress/gutenberg/pull/49018 to work, kses needs to support a variety of grid values.

For the current implementation only repeat and auto-fill are strictly necessary, but this may change in the future so it might be worth considering support for auto-fit, max-, min- and fit-content. The grid spec is getting built out fast, and soon there will also be good browser support for subgrid, with masonry following behind. See ​https://developer.mozilla.org/en-US/docs/Web/CSS/grid-template-columns for a list of possible values.

Trac ticket: https://core.trac.wordpress.org/ticket/58551

Adds support for the CSS repeat function.

The Trac ticket mentions support for auto-fit and -fill, but in testing I realised those are already implicitly supported so no changes needed there.

To test this change in the front end, apply the patch on top of #4625 and follow testing steps from that PR.

Trac ticket: https://core.trac.wordpress.org/ticket/58551

Adds support for the CSS repeat function.

The Trac ticket mentions support for auto-fit and -fill, but in testing I realised those are already implicitly supported so no changes needed there.

To test this change in the front end, apply the patch on top of #4625 and follow testing steps from that PR.

LGTM

Not sure about the removal of the aria-* attributes (seems like a typo/accidental)?

Definitely accidental! I wonder how it happened, I didn't edit those lines manually 🤔

[peterwilsoncc]

In 55944:

KSES: Add support for CSS repeat() function.

Introduces support for the CSS repeat() function to support complex grid layouts.

Props isabel_brison, azaozz.
Fixes #58551.

Committed in r55944 / ​https://github.com/WordPress/wordpress-develop/commit/2b7f8654171ba19188bb25b8226a93fcca9ed0c8