Split from {T348019}.
**Background**
In working on porting our stream parser to CodeMirror 6, I discovered that it's not very simple to apply line-level styles. Right now, all things that define the "MediaWiki language" are confined to the stream parser. In CodeMIrror 6, we need to use [[ https://codemirror.net/examples/decoration/ | Decorations ]] and apply them to the `EditorView`, which isn't available within the scope of the stream parser code. This means while possible, it's very difficult to replicate the same behaviour in CodeMirror 6.
That said, I can't seem to figure out how the section heading styles are even getting applied in CodeMirror 5. Normally this is done with [[ https://codemirror.net/5/doc/manual.html#addLineClass | doc.addLineClass() ]], but I see we're not using that method. Evidently there some magic somewhere that changes tokens prefixed with `line-` into a actual line-level style. I.e. at https://w.wiki/8Db7 we have `mw-section-header line-cm-mw-section-N`, but this actually gets outputted as just `mw-section-header`, and `cm-mw-section-N` somehow gets applied to the `.CodeMirror-line`.
I asked a few editors and they all seemed in favor of keeping the section heading styling if possible. This task is to further investigate making this happen.
**Acceptance criteria**
Level 1, 2 and 3 section headings should be in bold, and have incrementally larger/smaller font sizes, just as it does currently with CodeMirror 6.
For visual reference, we want this:
{F41522408}
and not this:
{F41522414}