Custom renderers cleanup - erb

See gitlab-com/www-gitlab-com#8343 (closed)

Merge request plan:

Source content anti-pattern fixes:

• MR1: source anit-pattern context fix gitlab-com/www-gitlab-com!57748 (merged)
• MR2: source anti-pattern content fix gitlab-com/www-gitlab-com!57761 (merged)

Source code fixes:

Original plan

This approach has been superseded by a pre-processing templater approach

• MR1: applies the pre approach at the paragraph level !38426 (closed)
• [-] MR2: handles escaping (may be able to use https://github.com/nhn/tui.editor/blob/6bcec75c69028570d93d973aa7533090257eaae0/libs/to-mark/src/renderer.js#L422)
• [-] MR3: handles the isComplex tree walking scenario (inline matches such as anchor links)
  • Possibly resolve with getChildrenText() -> regex matches array for ERB -> matches.map() -> using the lineOffsets to prepend indentation without walker(). Doesn't work

Pre-processing plan

• MR1: initial ERB syntax (blocks) !38694 (merged)
• MR2: follow-up ERB syntax (inline + HTML blocks) !38791 (merged)
• MR3: exclude pre-existing codeblocks !38834 (merged)