[add] new parser

[Mc-Zen]

This PR adds a new parser to tidy. This is part of a plan to change the format of doc-comments, see #32. The old parser might still be available as an alternative for a while.

/// Description
/// -> float
#let my-function(
  /// Parameter x
  /// -> float
  x: 999,

  /// Parameter y
  /// -> float
  x: 420,
) = ...