Customize: Prevent scrolling `custom_css` textarea to top when pressing tab.

Props tyxla, sstoqnov for testing, celloexpressions for testing.
See #38667.
Fixes #39134.

Built from https://develop.svn.wordpress.org/trunk@39557


git-svn-id: https://core.svn.wordpress.org/trunk@39497 1a063a9b-81f0-0310-95a4-ce76da25c4cd