Use JSON output and create annotations

[Benjamin Wuethrich]

With shellcheck --format=json1, output looking something like

{
  comments: [
    {
      "file": "filename",
      "line": lineNumber,
      "column": columnNumber,
      "level": "severitylevel",
      "code": errorCode,
      "message": "warning message"
    },
    ...
  ]
}

can be generated, and used to create annotations directly on the offending file location.