fix(GLTFCesiumRTCExtension.d.ts): implements THREE.js's GLTFLoaderPlugin

This typing is necessary to register this extension on the three.js GLTFLoader.

src/plugins/three/gltf/GLTFCesiumRTCExtension.d.ts

@@ -1,4 +1,6 @@
-export class GLTFCesiumRTCExtension {
+import { GLTFLoaderPlugin } from 'three/examples/jsm/loaders/GLTFLoader.js';
+
+export class GLTFCesiumRTCExtension implements GLTFLoaderPlugin {
 
 	name: 'CESIUM_RTC';