Add background-repeat CSS property to safe_style_css filter

[andrewserong]

To support extending both the style engine and background image block supports to include a toggle for background-repeat, let's add the background-repeat property to the safe_style_css filter, to ensure the CSS property can be output.

The proposed new control to allow toggling background repeat has landed in the Gutenberg plugin as of ​https://github.com/WordPress/gutenberg/pull/57005. Note that other PHP parts of that PR will be backported separately, to keep things contained.

This PR adds the background-repeat property to safe_style_css to support the missing background repeat property.

This is a prerequisite for adding in a background-repeat toggle to the background image support feature, which has landed in Gutenberg in: ​https://github.com/WordPress/gutenberg/pull/57005

Note: that PR includes other PHP changes that will be backported separately, so that this PR can just focus on the proposed change to kses.php and its tests.

To test, ensure the tests pass, or if you're using the Docker environment for wordpress-develop, run the following locally: npm run test:php /var/www/tests/phpunit/tests/kses.php

Trac ticket: https://core.trac.wordpress.org/ticket/60132

Thanks for giving this a quick look! I forgot to add the ticket number to the test that uses this data provider. Added in ​https://github.com/WordPress/wordpress-develop/pull/5814/commits/5fc84d2f977ce0bd3016660ceab2d03f664203d6

[SergeyBiryukov]

In 57228:

KSES: Add background-repeat to the list of safe CSS properties.

Follow-up to [45242], [46235].

Props andrewserong, ramonopoly, mukesh27.
Fixes #60132.

Thanks for the PR! Merged in r57228.