SkyMesh: Add isSkyMesh flag and deprecate isSky (#1899)

Author: Methuselah96

types/three/examples/jsm/objects/SkyMesh.d.ts

@@ -8,8 +8,13 @@ declare class SkyMesh extends Mesh<BoxGeometry, NodeMaterial> {
     sunPosition: UniformNode<Vector3>;
     upUniform: UniformNode<Vector3>;
 
+    /**
+     * @deprecated
+     */
     readonly isSky: true;
 
+    readonly isSkyMesh: true;
+
     constructor();
 }