Breaking changes to Document & createNode APIs

[eemeli]

This PR combines a few closely related changes, and is intended to streamline how new documents and nodes are created.

new YAML.Document(value, options?)

The Document constructor gets a new first argument value. This allows for much more ergonomic document creation:

// before
const doc = new YAML.Document()
doc.setSchema()
doc.contents = doc.schema.createNode({ some: 'value' })

// after
const doc = new YAML.Document({ some: 'value' })

The doc.schema is now also left uninitialised only if value === undefined, and is automatically set if createNode() or createPair() are called.

doc.createNode(value, options?)

The methods YAML.createNode() and doc.schema.createNode() are replaced by a single doc.createNode(). Their non-value arguments are also collected into an options object:

// before
const doc = YAML.createDocument({ customTags: ['omap'] })
doc.setSchema()
const seq = YAML.createNode([1, 2, 3])
const omap = doc.schema.createNode([['foo', 13], ['bar', 42]], true, '!!omap')

// after
const doc = YAML.createDocument(null, { customTags: ['omap'] })
const seq = doc.createNode([1, 2, 3])
const omap = doc.createNode([['foo', 13], ['bar', 42]], { tag: '!!omap' })

Unlike its precedents, doc.createNode() is able to support circular references, which were previously only supported by YAML.stringify().

doc.createPair(key, value, options?)

The method doc.schema.createPair() is replaced by doc.createPair(), and also uses an options bag instead of named positional arguments.

// before
const doc = YAML.createDocument()
doc.setSchema()
const pair = doc.schema.createPair('key', 'value')

// after
const doc = YAML.createDocument()
const pair = doc.createPair('key', 'value')

Internally, doc.createPair() is just sugar around two separate doc.createNode() calls and the Pair constructor.

[eemeli]

Rebase to account for changes to master.