Add commentString option

[eemeli]

Related: #322, cc @leipert

With this, it's possible to define a custom stringifier for comments, using a pretty simple API:

type ToStringOptions = {
  /**
   * Comment stringifier.
   * Output should be valid for the current schema.
   *
   * By default, empty comment lines are left empty,
   * lines consisting of a single space are replaced by `#`,
   * and all other lines are prefixed with a `#`.
   */
  commentString?: (comment: string) => string,
  ...
}

This has at least three potential uses:

1. Folding long comment lines.
2. Ensuring that a space always follows the #.
3. Using the library to emit non-YAML comments, such as /* foo */.

The default value is rather compact:

const commentString = (comment: string) => comment.replace(/^(?!$)(?: $)?/gm, '#')