**Background**
In CodeMIrror 6, the "proper" way to introduce syntax highlighting for a language is to write a [[ https://codemirror.net/examples/lang-package/ | language pack ]]. Multiple people have investigated this and concluded it would be a very daunting challenge, so we have instead settled on porting our existing [[ https://github.com/wikimedia/mediawiki-extensions-CodeMirror/tree/1a19f48c7009bfa7b5e7cc27361d2cf944daf65f/resources/mode/mediawiki | MediaWiki mode ]] to CodeMirror 6's [[ https://github.com/codemirror/legacy-modes | stream-parser interface ]]. Efforts to build a custom parser for MediaWiki should be tracked in a separate task.
**Acceptance criteria**
* With `$wgCodeMirrorV6` set to `true` (following T317243), or by passing in the `cm6enable=1` in the URL query string, we should now have actual syntax highlighting
* All syntax highlighting should work identical to as what's currently in prod, with exception of extensions (i.e. `<ref>`, `<phonos>`, etc.) (T348684)
* Add [[ https://codemirror.net/docs/ref/#h_bracket_matching | bracket matching ]] to keep feature parity with CodeMirror 5