Add comments to the AST walker

[TwitchBronBron]

Now that we removed CommentStatement from the AST as nodes, we need a way to discover all comments when walking the AST.

Maybe a new WalkMode option that includes comments?

[markwpearce]

Goal for usage:

file.ast.walk(createVisitor({
   CommentToken: (comment) -> {
       // this is a comment!
       
   }),
   { walkMode: WalkMode.visitAllRecursive & WalkMode.VisitComments }
);