Implement the std::fmt::Debug trait.

[DaveMcEwan]

• Based on the Display trait, but includes whitespace.
• The format of each line is slightly different, with @line:123 appearing before the token's string.
  • This is to reduce the chances of grep-based tools getting confused.
  • This also makes it easier to display whitespace/comments.
• No additional testcases are added, same as Display.
• Tested via local build in svlint development.
• Immediately, useful for debugging style_* rules in svlint. Later, intended to be used debugging sv_parser_pp::preprocess().