Add missing YAML 1.1 collections: omap, pairs, set

[eemeli]

This adds a new instance method createNode(item, wrapScalars, tag) to the document schema, along with adding the following tags to the yaml-1.1 schema:

!!omap is an ordered key:value mapping, represented in JS as a Map and in YAML as a sequence of pairs:

const src = '!!omap [ a: 1, b: 2, c: 3 ]'
const map = YAML.parse(src, { version: '1.1' }) // Map { 'a' => 1, 'b' => 2, 'c' => 3 }

YAML.stringify(map, { version: '1.1' }) // '!!omap\n- a: 1\n- b: 2\n- c: 3\n'
YAML.stringify(map, { version: '1.2' }) // 'a: 1\nb: 2\nc: 3\n'

!!pairs is very similar to !!omap, but allows duplicate keys. It does not have an exactly corresponding JS equivalent, and so is represented as an array of { key: value } objects:

const src = '!!pairs [ a: 1, b: 2, a: 3 ]'
const pairs = YAML.parse(src, { version: '1.1' }) // [ { a: 1 }, { b: 2 }, { a: 3 } ]

// Array of pairs is by default parsed as !!seq
YAML.stringify(pairs, { version: '1.1' }) // '- a: 1\n- b: 2\n- a: 3\n'

// To create a new !!pairs, you'll need a document with a schema
const doc = new YAML.Document({ version: '1.1' })
doc.setSchema()
doc.contents = doc.schema.createNode(pairs, false, '!!pairs')
String(doc) // '!!pairs\n- a: 1\n- b: 2\n- a: 3\n'

Both !!omap and !!pairs support construction from either a Map, an array of [ key, value ] tuples, or an array of { key: value } objects.

!!set is an unordered set of unique values. In JS it's a Set and in YAML a mapping with null values:

const src = '!!set { a, b, c }'
const set = YAML.parse(src, { version: '1.1' }) // Set { 'a', 'b', 'c' }

YAML.stringify(set, { version: '1.1' }) // '!!set\n? a\n? b\n? c\n'
YAML.stringify(set, { version: '1.2' }) // '- a\n- b\n- c\n'

To use these collections with e.g. the core schema, they may be included as custom tags:

import omap from 'yaml/types/omap'
import pairs from 'yaml/types/pairs'
import set from 'yaml/types/set'

YAML.defaultOptions.tags = [ omap, pairs, set ]
YAML.parse('!!set { a, b, c }') // Set { 'a', 'b', 'c' }

With these additions, yaml should now directly support all YAML 1.1 tags, except for !!value and !!yaml, which are unlikely to get implemented unless someone comes up with a really convincing usage scenario.