CodeMirror 6 compatiblity

Hello! Per [T357794](https://phabricator.wikimedia.org/T357794) on Wikimedia's bug tracker, this extension appears to integrate or adjust the styles of CodeMirror 5. This is slated to be replaced by [CodeMirror 6](https://www.mediawiki.org/wiki/Extension:CodeMirror) in MediaWiki 1.46.

I see this repo targets `.CodeMirror` in CSS, which will [no longer work](https://www.mediawiki.org/wiki/Extension:CodeMirror#CSS) after MW 1.46. I've been making patches/PRs to repos where it's clear what to do, but for this one I'm not sure. Note we already have dark mode styles in CodeMirror 6. You apply it, you can add `.skin-theme-clientpref-night` to the `` element, like Vector 2022. Eventually (hopefully MW 1.47), we'll offer dark mode as a [theme](https://phabricator.wikimedia.org/T163533), if that makes it any easier.

Please see the [migration guide](https://www.mediawiki.org/wiki/Extension:CodeMirror#Migration_guide) for more general information on the CodeMirror 5 deprecation. Let me know if you've any questions or need any help!