Customize: Prevent single quotes (apostrophes) in `custom_css` values from...

Customize: Prevent single quotes (apostrophes) in `custom_css` values from unexpectedly causing false positives for unbalanced character validation errors.

See #39218, #35395.
Fixes #39198.

Built from https://develop.svn.wordpress.org/trunk@39559


git-svn-id: https://core.svn.wordpress.org/trunk@39499 1a063a9b-81f0-0310-95a4-ce76da25c4cd