Change

• 8fe6893 Update @types/nlcst, @types/unist, utilities
  migrate: update too
• ad2d932 Change to require Node.js 16
  migrate: update too
• 6ce04d2 Change to use exports
  migrate: don’t use private APIs
• 1d96ee6 Change to use undefined for doc field
  migrate: expect undefined

Full Changelog: 6.0.2...7.0.0