Add TypeScript definitions for some utils

examples/jsm/utils/BufferGeometryUtils.d.ts

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+import { BufferAttribute, BufferGeometry } from '../../../src/Three';
+
+export namespace BufferGeometryUtils {
+	export function mergeBufferGeometries(geometries: BufferGeometry[]): BufferGeometry;
+	export function computeTangents(geometry: BufferGeometry): null;
+	export function mergeBufferAttributes(attributes: BufferAttribute[]): BufferAttribute;
+}

examples/jsm/utils/GeometryUtils.d.ts

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+/**
+ * @deprecated
+ */
+export namespace GeometryUtils {
+	/**
+	 * @deprecated Use {@link Geometry#merge geometry.merge( geometry2, matrix, materialIndexOffset )} instead.
+	 */
+	export function merge(geometry1: any, geometry2: any, materialIndexOffset?: any): any;
+	/**
+	 * @deprecated Use {@link Geometry#center geometry.center()} instead.
+	 */
+	export function center(geometry: any): any;
+}

examples/jsm/utils/SceneUtils.d.ts

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+import { Geometry, Material, Object3D, Scene } from '../../../src/Three';
+
+export namespace SceneUtils {
+	export function createMultiMaterialObject(geometry: Geometry, materials: Material[]): Object3D;
+	export function detach(child: Object3D, parent: Object3D, scene: Scene): void;
+	export function attach(child: Object3D, scene: Scene, parent: Object3D): void;
+}