6.0.0

Change

• 900cf9a Update @types/mdast
  migrate: update too
• 79d4d61 Change to require Node.js 16
  migrate: update too
• 9e02a5b Change to use export map
  migrate: don’t use private APIs
• 4a93553 Change to return undefined, not null
  migrate: expect undefined

Full Changelog: 5.1.2...6.0.0