PSRule-vscode/syntaxes/rule.json

{
  "scopeName": "PSRule-powershell-rule",
  "injectionSelector": "L:source.powershell",
  "patterns": [
    {
      "include": "#rule-keyword"
    }
  ],
  "repository": {
    "rule-keyword": {
      "match": "(?<=^)(?i)(rule)(?=\\s+)",
      "name": "storage.type.powershell"
    }
  }
}