Theme Editor: Validate files are editable based on their relative filenames,...

Theme Editor: Validate files are editable based on their relative filenames, rather than full file path.

This fixes theme editing on Windows platforms where `validate_file()` will return `2` on a full file path.

Fixes #42609.

Built from https://develop.svn.wordpress.org/trunk@42244


git-svn-id: https://core.svn.wordpress.org/trunk@42073 1a063a9b-81f0-0310-95a4-ce76da25c4cd