Add collectionStyle option

[eemeli]

Add to ToString options:

type ToStringOptions = { ...
  /**
   * Enforce `'block'` or `'flow'` style on maps and sequences.
   * Empty collections will always be stringified as `{}` or `[]`.
   *
   * Default: `'any'`, allowing each node to set its style separately
   * with its `flow: boolean` (default `false`) property.
   */
  collectionStyle?: 'any' | 'block' | 'flow'
}

This should help in particular with the production of JSON-ish output, where block collections are not supported.