Streamline whitespace and comment handling

[oliverklee]

Which steps do we need to take in which order, and for which branches?

• [BUGFIX] Fix comment parsing to support multiple comments #672
  • main
  • v8.x
• [TASK] Mark parsing-internal classes and methods as @internal #674 (which will allow us to courageously refactor the parsing code)
  • main
  • v8.x
• #CSS comments lead to exception #680
  • main
  • v8.x

@ziegenberg Which other steps for which branches do you think we should take? If you comment here, I'll update the checklist accordingly.

Hopefully this discussion and checklist will help us get the moving target move a bit less, and create a shared understanding of the plan. :-)