feat: Add $schema property

[pke]

VS Code supports code completion for JSON files based on JSON Schema.

Code completion in VS Code picks up a schema for any JSON file if the schema is
specified as "$schema": "<URL to schema>".

It automatically picks up certain JSON Schemas by file name (ie: package.json) and you can configure your own associations via (workspace) settings.json like this:

"json.schemas": [
  {
    "fileMatch": [
      "resume.json"
    ],
    "url": "http://json.schemastore.org/resume"
  }
]

Alternatively VS Code supports the $schema key in JSON files itself.

So in a resume.json we could add the JSON Resume schema to get code completion without touching the VS Code settings:

{
  "$schema": "http://json.schemastore.org/resume",
}

So please JSON Schema authors allow the specification of the schema in your JSON Schema like this:

{
  "properties": {
    "$schema": {
      "title": "JSON Schema to use",
      "description": "Can be a local schema or a remote one (ie from http://schemastore.org)",
      "oneOf": [
        {
          "type": "string",
          "format": "uri"
        },
        {
          "type": "string"
        }
      ]
    },
  }
}

XML had a similar feature where you could specify an XSD in the XML pre-processing tags to validate the syntax and data (to some extent) of the XML.