Parser Progression

- Finished most of the logic for code block (incidentally,
  comment blocks as well). Only thing missing is to strip
  the last line break in the parsed text.

- `.shift()`, `.shiftUntil()`, and `.shiftUntilPast()`
  now return the token index they have stopped in the
  lexical token stream.

  Cleaned up the code base significantly using this change,
  especially for stripping text.

- Added a guard in `parseBlock()` to return nothing,
  in case there's nothing left in the token stream.