Toolbar: Show command palette admin bar item on mobile.

* The keyboard shortcut is hidden in favor of a search icon on mobile.
* The background color of the `KBD` element is made transparent on non-mobile.
* The admin bar item is skipped from being added if the `wp-core-commands` script is not enqueued.

Developed in https://github.com/WordPress/wordpress-develop/pull/11222

Follow-up to r61912.

Props sabernhardt, westonruter, ellatrix, mukesh27.
See #64672.


git-svn-id: https://develop.svn.wordpress.org/trunk@61979 602fd350-edb4-49c9-b593-d223f7449a82