Expose elm-format as a Haskell library

[avh4]

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• [x] build haddock docs https://github.com/avh4/elm-format/pull/805
• [ ] review what the public API looks like
  • [ ] AST
  • [ ] to/from AST functions
    • [ ] Text
    • [ ] ByteString
    • [ ] ByteString.Builder
  • [ ] higher-level IO (or MonadIO) API?
  • [ ] (maybe) recursion-schemes API
• [ ] TBD... make any necessary changes to the API
  • renaming
    • [ ] AST should go under ElmFormat.AST.ElmCompiler.V0_19
  • [ ] CommandLine.* stuff doesn't belong in this package?
  • [ ] ideally, Box stuff (take latest version from Gren) doesn't belong in this package?
• [ ] loosen version bounds
  • [ ] allow ghc 9.0
  • [ ] allow ghc 9.4
  • [ ] others, TBD...
• [ ] get hackage account
• [ ] publish first version