Add a D2 example

[Guillaume Grossetie]

D2 Parser: {
  shape: class

  # Default visibility is + so no need to specify.
  +reader: io.RuneReader
  readerPos: d2ast.Position

  # Private field.
  -lookahead: "[]rune"

  # Protected field.
  # We have to escape the # to prevent the line from being parsed as a comment.
  \#lookaheadPos: d2ast.Position

  +peek(): (r rune, eof bool)
  rewind()
  commit()

  \#peekn(n int): (s string, eof bool)
}

"github.com/terrastruct/d2parser.git" -> D2 Parser