test: added validation of fcpxml against dtd provided by apple.

You can find the DTD here https://developer.apple.com/documentation/professional_video_applications/fcpxml_reference/document_type_definition

Author: dk949

package.json

@@ -38,6 +38,7 @@
     "@typescript-eslint/parser": "^5.2.0",
     "eslint": "^8.1.0",
     "mocha": "^9.1.2",
+    "node-libxml": "^4.1.2",
     "ts-node": "^10.2.1",
     "typescript": "^4.3.4"
   }