Add support for comments

Comments are not implemented right now but can be very simple:

• Comments start with ;, like Lisp
• Start of line, or end of line
• You can trim it as first step before tokenizing code (this way, the parser/ast are ignorant of comments)
• Prefer not using regex, but if you have to, it's good enough (because in the future, we'll be happy to get rid of the regex crate, which only does tokenization for now)