fix: declare getMetaDataSymbol() as returning symbol | string

spec/typings/typings-test.ts

@@ -41,6 +41,18 @@ const parsed = parser.parse(XML);
 
 console.log(!!parsed);
 
+// The metadata key indexes parsed nodes, so it has to be a primitive property key, not a boxed `Symbol`.
+const metaDataKey: PropertyKey = XMLParser.getMetaDataSymbol();
+const metaDataNode = new XMLParser({ captureMetaData: true }).parse(XML).SomeElement;
+
+console.log(metaDataNode[metaDataKey], metaDataNode[XMLParser.getMetaDataSymbol()]);
+
+// Without `Symbol` in the environment the key falls back to a plain property name.
+type StringKeyAccepted = string extends ReturnType<typeof XMLParser.getMetaDataSymbol> ? true : false;
+const stringKeyAccepted: StringKeyAccepted = true;
+
+console.log(stringKeyAccepted);
+
 const buildOpts: XmlBuilderOptions = {};
 
 const builder = new XMLBuilder(buildOpts);