The CodeEditor extension should support activating the editor on JS and CSS pages on a MediaWiki wiki. I believe this is a matter of checking isJsorCssPage() or something similar. I also believe this is relatively straightforward to implement, so I'm marking this as easy.
I think there's a reasonable expectation that enabling this extension will affect JS and CSS pages on the wiki (plus the editor can easily be toggled off), but I guess you could put this particular feature behind a global configuration variable, if you wanted to.
Version: unspecified
Severity: enhancement