Add a toJS(doc, options?) method to nodes

[eemeli]

Fixes #449
CC @MikeRalphson

This makes it easy to JS-ify parts of a YAML document, for example like this:

import { Alias, parseDocument } from 'yaml'

const doc = parseDocument(`
hello: &a
  world:
    message: Hello, world
outputs: *a
`)

doc.getIn(['hello', 'world']).toJS(doc)
// { message: 'Hello, world' }

new Alias('a').toJS(doc)
// { world: { message: 'Hello, world' } }

The doc argument is required here to use the right schema, and for resolving aliases. A second options argument is also supported, with the same options as provided for doc.toJS().

Some of the utility functions previously defined in nodes/Node.ts need to be split off into their own nodes/identity.ts here to avoid circular dependencies.