function markdownLineEnding(code) {
  return code !== null && code < codes.horizontalTab
}
Check whether a character code is a markdown line ending.

A **markdown line ending** is the virtual characters M-0003 CARRIAGE RETURN
LINE FEED (CRLF), M-0004 LINE FEED (LF) and M-0005 CARRIAGE RETURN (CR).

In micromark, the actual character U+000A LINE FEED (LF) and U+000D CARRIAGE
RETURN (CR) are replaced by these virtual characters depending on whether
they occurred together.

@param {Code} code
  Code.
@returns {boolean}
  Whether it matches.