feat: refactor control classes to use readonly for target property (#370)

Author: Kosuke-KIDO-datalabs

src/controls/ArcballControls.ts

@@ -190,7 +190,7 @@ class ArcballControls extends EventDispatcher {
   public minZoom: number
   public maxZoom: number
 
-  private target: Vector3
+  readonly target: Vector3
   private _currentTarget: Vector3
 
   private _tbRadius: number

src/controls/FirstPersonControls.ts

@@ -49,7 +49,7 @@ export class FirstPersonControls extends EventDispatcher {
 
   private lookDirection = new Vector3()
   private spherical = new Spherical()
-  private target = new Vector3()
+  readonly target = new Vector3()
 
   constructor(object: Camera, domElement?: HTMLElement | null) {
     super()