[FEATURE] --json flag for structured output

Add JSON output format flag for machine-readable validation results with metadata and error details.

Example output:
```JSON
{
  "valid": true,
  "version": 2,
  "file": "package.apg",
  "metadata": {
    "name": "astrum",
    "version": "1.3.2",
    "type": "application"
  },
  "errors": [],
  "warnings": []
}
```