SkyMesh: Add isSkyMesh flag and deprecate isSky (#32263)

Co-authored-by: Michael Herzog <michael.herzog@human-interactive.org>

examples/jsm/objects/SkyMesh.js

@@ -88,8 +88,18 @@ class SkyMesh extends Mesh {
 		 * @type {boolean}
 		 * @readonly
 		 * @default true
+		 * @deprecated Use isSkyMesh instead.
 		 */
-		this.isSky = true;
+		this.isSky = true; // @deprecated, r182
+
+		/**
+		 * This flag can be used for type testing.
+		 *
+		 * @type {boolean}
+		 * @readonly
+		 * @default true
+		 */
+		this.isSkyMesh = true;
 
 		// Varyings