Module scope docstring support

[ryyppy]

There should be dedicated docstring parsing / printing for module level docs.

Example:

// Utils.resi
/**
This module contains convenience functions for strings, arrays and dates.
(this is a docstring attached to the toplevel scope of the `Utils` module)
*/

/**
Date utils (This docstring should apply to the submodule `Date`)
*/
module Date = {
  /** Formats a date in a human-readable way (YYYY-MM-DD) (This is a normal value doc string) */
  let formatDate: (Js.Date.t) => string;
}

We are currently using @ns.doc as the internal presentation of value-level docstrings, so we could think about using a @@ns.doc decorator for module level usage.

Probably worth noting that ocamldoc used a @@ocaml.text attribute for module level documentation. Needs more investigation why that is, but ideally we'd like to keep the number of attribute names low for simplicity.