Add background-repeat to safe_style_css

[andrewserong]

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: WordPress/gutenberg#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

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[ramonjd]

I ran the tests.

The tests passed.

👍🏻

[andrewserong]

Thanks for giving this a quick look! I forgot to add the ticket number to the test that uses this data provider. Added in 5fc84d2

[github-actions]

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[SergeyBiryukov]

Thanks for the PR! Merged in r57228.