feat: TS type information for remark-parse

[mike-north]

NOTE: This branch is based on top of #383

This PR adds ambient type information and tests for the remark-parse package

Consumers of this library who benefit from TypeScript type information can do stuff like

import * as remarkParse from 'remark-parse'
import {RemarkParseOptions} from 'remark-parse/types'
import * as unified from 'unified'

/**
 * 👇 type-checking against the options object
 */
const parseOptions: Partial<RemarkParseOptions> = {
  gfm: true,
  pedantic: true
}

unified().use(remarkParse, parseOptions)

/**
 * 👇 type-checking against the options object
 */
const badParseOptions: Partial<RemarkParseOptions> = {
  gfm: 'true' // 🚨
}

[Rokt33r]

I've merged this pr into #383