Quick-fix: Normalize UTF-8 charset slug detection.
In the merge of [58417] the new file was missed. This commit adds the missing required file.
Developed in ���https://github.com/WordPress/wordpress-develop/pull/6535
Discussed in https://core.trac.wordpress.org/ticket/61182
Fixes #61182.
Follow-up to [58147].
Props dmsnell, jonsurrell.