Sphere: Add isSphere flag. (#715)

Author: Methuselah96

types/three/src/math/Sphere.d.ts

@@ -6,6 +6,11 @@ import { Matrix4 } from './Matrix4.js';
 export class Sphere {
     constructor(center?: Vector3, radius?: number);
 
+    /**
+     * Read-only flag to check if a given object is of type {@link Sphere}.
+     */
+    readonly isSphere: true;
+
     /**
      * @default new Vector3()
      */