perf: use ValueListBuilder for PrivatePrintLeadingTrivia

[TimothyMakkison]

Added ValueListBuilder to PrivatePrintLeadingTrivia saving 0.55 MB

• Added ValueListBuilder.Pop - this is an unmodified copy of microsofts Pop
• Added my own version of Insert(int index, T item)
• Added ValueListBuilder.ToList this is because CompilationUnit needs the resulting IList<Doc> contents to be an actual List<Doc> instead of an array.
  • I could modify CompilationUnit.Print to not need a mutable list WDYT.

This isn't a massive saving I can close this if you like. I suspect this is more impactful for projects with a lot of comments and summaries though.

[belav]

I'm wondering if keeping it is worth it to try to keep Token consistently using ValueListBuilder.

Besides this, multiline raw strings also make use of List<Doc>, I'm not sure how much they would benefit from it.

In theory I think CompilationUnit could set things to Doc.Null instead of removing them. I think it is pretty rare that it actually ends that code block, so it may be worth it to avoid the ToList call. And some day if I can find the time I did want to try to redo all of this comment logic, which would probably also get rid of the need for ToList.