7.0.0

Change

• a541a47 Update @types/mdast
  migrate: update too
• b024d88 Change to require Node.js 16
  migrate: update too
• 09b1c2e Change to use export map
  migrate: don’t use private APIs
• 2e3ff5b Change to return undefined, not null
  migrate: expect undefined

Full Changelog: 6.1.1...7.0.0