Build/Test Tools: Add banner to RTL CSS and minified JS files.

Patches occasionally come in on generated files. We should be kind to new contributors and give them a hint that these files are auto-generated.

This is a follow-up to [41271], which added the banner to minified CSS files.

Fixes #48424. See #30666.
Built from https://develop.svn.wordpress.org/trunk@46589


git-svn-id: https://core.svn.wordpress.org/trunk@46386 1a063a9b-81f0-0310-95a4-ce76da25c4cd