Project: Add CODEOWNERS entries for ui and theme packages

[aduth]

What?

Adds a pair of entries to the CODEOWNERS file for the new @wordpress/ui and @wordpress/theme packages, currently assigned to the @WordPress/gutenberg-components GitHub team.

Why?

• To improve collective awareness to changes ongoing in these early-stage packages
• As a basis to extend for others who may be interested to review changes in these packages

Testing Instructions

Verify correct syntax and accurate assignee naming in the CODEOWNERS file.

Verifying review assignment is only possible after these changes are merged.

[github-actions]

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If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[fabiankaegy]

I'd also gladly join the @WordPress/gutenberg-components team / get added manually to these packages as a code owner to stay up to date / help review work :)

[aduth]

I'd also gladly join the @WordPress/gutenberg-components team / get added manually to these packages as a code owner to stay up to date / help review work :)

That sounds awesome @fabiankaegy ! I'll plan to connect with you on Slack to see what'll work best.