3.0.0

Refactor project to monorepo

• Add new retext-latin, retext-stringify packages;

• Internalise retext-english, retext-dutch packages;

• Update dependencies, dev-dependencies, most importantly
  unified, which provides a new better interface
  (unifiedjs/unified@a070ad0).

  Most importantly, do retext().process instead of
  retext.process, and process() now returns a
  VFile (can be stringified with toString()).