Split from T348019: Port MediaWiki mode to CodeMirror 6 stream-parser.
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 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.
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 5.
For visual reference, we want this:
and not this:
Additionally, <nowiki> and <pre> tags should have line-level styling applied to full lines in between the tags, i.e.:
instead of:
