fix: improve a11y of sortable keys

[HardeepAsrani]

Summary

This makes the sortable items in Customizer and Post Editor keyboard-ready. You can take the focus to the dragger, click space and then press up/down key to move the item, and then space again to end selection.

Will affect visual aspect of the product

No

Screenshots

Test instructions

• Make sure the items are moving properly with the keyboard.
• And then make sure they continue to work as before without any issues.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/neve-pro-addon/issues/3065.

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