4.0.0

Change

• ae6c215 Update @types/mdast
  migrate: update too
• 07c45fa Change to require Node.js 16
  migrate: update too
• a5ac22a Change to use export map
  migrate: don’t use private APIs
• 45cb3fc Change to return undefined
  migrate: expect `undefined

Full Changelog: 3.1.1...4.0.0