Add Grace_json_renderer

[WardBrian]

Closes #82.

For testing, I am re-using the ansi renderer tests to a large degree. Obviously they could be simplified, since the edge cases are much more limited in this renderer.

Example:

$ dune exec -- ./examples/main.exe --json
{
  "error_code": "E001",
  "severity": "error",
  "message": "`match` cases have incompatible types",
  "notes": [],
  "labels": [
    {
      "range": {
        "source": "fizz.ml",
        "start": { "line": 7, "column": 14 },
        "end": { "line": 8, "column": 1 }
      },
      "priority": "primary",
      "message": "expected `[> `Buzz | `Fizz | `Fizz_buzz]`, found `int`"
    },
    {
      "range": {
        "source": "fizz.ml",
        "start": { "line": 3, "column": 4 },
        "end": { "line": 8, "column": 1 }
      },
      "priority": "secondary",
      "message": "`match` cases have incompatible types"
    },
    {
      "range": {
        "source": "fizz.ml",
        "start": { "line": 4, "column": 14 },
        "end": { "line": 5, "column": 1 }
      },
      "priority": "secondary",
      "message": "this is found to be of type `[> `Fizz_buzz]`"
    },
    {
      "range": {
        "source": "fizz.ml",
        "start": { "line": 5, "column": 14 },
        "end": { "line": 6, "column": 1 }
      },
      "priority": "secondary",
      "message": "this is found to be of type `[> `Fizz]`"
    },
    {
      "range": {
        "source": "fizz.ml",
        "start": { "line": 6, "column": 14 },
        "end": { "line": 7, "column": 1 }
      },
      "priority": "secondary",
      "message": "this is found to be of type `[> `Buzz]`"
    }
  ]
}

[johnyob]

Thanks for the contribution! 🙏

Just a few comments (mostly style / nits), the logic all looks correct 🚀

[johnyob]

I'd prefer to avoid duplicating test data (we should probably start putting these code snippets in dedicated files anyway), but that can be done in a separate PR :)

[WardBrian]

@johnyob thanks for the comments. I took all your suggestions minus the deduplication of tests for now.

[johnyob]

lgtm 🚀

Seems like you need to make CI happy (likely your commit message format). After that we can merge :)

[WardBrian]

Thanks @johnyob -- I squashed everything into one feat commit which should hopefully please the CI

[jonsterling]

One thing I would like to raise is that AFAIK major language clients, such as that of Visual Studio Code, only properly support UTF-16 offsets. So integrating Grace with the language server protocol will in practice require switching to UTF-16 offsets, because VS Code is the de facto 'canonical' instance of LSP.

Does it seem like there is a chance of making this change, or of at least providing this option? /cc @kentookura.