Reuse 'data-set-focus' to simulate twentytwenty keyboard focus

[schlessera]

Summary

Changes the toggle emulation logic to reuse TwentyTwenty's data-set-focus attribute and manually set keyboard focus when triggering a toggle.

This ensures we correctly set keyboard focus on the Close button when opening the menu modal.

Fixes #3618

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[westonruter]

Nice work.