Add AstNode visitor function for createVisitor

[markwpearce]

Allows:

const { ast } = Parser.parse(`
    sub main()
       'comment
       print "hello"
    end sub
`);
ast.walk(createVisitor({
    CommentToken: (comment, parent, owner, key) => {
        console.log(comment.text); // prints "'comment"
    }
}), {
    walkMode: WalkMode.visitAllRecursive | WalkMode.visitComments
});

[markwpearce]

Fixes #1128

[markwpearce]

Looks great.