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.
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:
{F41522408}
and not this:
{F41522414}
Additionally, `<nowiki>` and `<pre>` tags should have line-level styling applied to //full// lines in between the tags, i.e.:
{F41649426}
instead of:
{F41649428}