Extract player entity setup into a Player class

Moves the capsule/physics/renderable/PLAYER setup out of
overworldscene.js's inline init() into assets/scripts/Player.js so the
scene module just instantiates it.

assets/scripts/Player.js

@@ -0,0 +1,26 @@
+// Copyright (c) 2026 Dominic Masters
+//
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+// Player: dynamic capsule body, moved relative to the camera by PLAYER's
+// own update callback (see entityplayer.c).
+class Player {
+  constructor() {
+    this.entity = new Entity();
+
+    this.position = this.entity.add(POSITION);
+    this.position.setLocalPosition(0.0, 2.0, 0.0);
+
+    this.physics = this.entity.add(PHYSICS);
+    this.physics.setShape(PHYSICS_SHAPE_CAPSULE, 0.5, 0.5);
+
+    this.renderable = this.entity.add(RENDERABLE);
+    this.renderable.setMesh(0, MESH_CAPSULE);
+    this.renderable.setColor(0, 0, 255, 255);
+
+    this.entity.add(PLAYER);
+  }
+}
+
+module.exports = Player;