cleanup: Use spread properties instead of Object.assign()

It's more concise and has been around long enough to embrace it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3196>

js/misc/params.js

@@ -25,6 +25,5 @@ export function parse(params = {}, defaults, allowExtras) {
         }
     }
 
-    let defaultsCopy = Object.assign({}, defaults);
-    return Object.assign(defaultsCopy, params);
+    return {...defaults, ...params};
 }