Create JSON-compatible errors

[JohnnyMorganz]

--output-format=json will now output all messages in JSON format.
Structure:

{
    "type": "error", // or "warn"/"info"/"debug" etc. and "parse_error"
    "message": "XYZ"
}

and if type == "parse_error" we also have a filename and location field like

{
    "type": "parse_error",
    "message": "XYZ",
    "filename": "src/test.lua",
    "location": {
        "start": 0, // byte offset from start of file
        "start_line": 0,
        "start_column": 0,
        "end": 0, // byte offsets from start of file
        "end_line": 0,
        "end_column": 0,
    }
}

Closes #453

[codecov]

Codecov Report

Merging #455 (6c0ef00) into master (11a4c8c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #455   +/-   ##
=======================================
  Coverage   96.44%   96.44%           
=======================================
  Files          14       14           
  Lines        5257     5257           
=======================================
  Hits         5070     5070           
  Misses        187      187           

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