Add TypeScript definitions for GLTFExporter

examples/jsm/exporters/GLTFExporter.d.ts

@@ -0,0 +1,7 @@
+import { Object3D } from '../../../src/Three';
+
+export class GLTFExporter {
+  constructor();
+
+  parse(input: Object3D, onCompleted: (gltf: object) => void, options: object): null;
+}