Refactor to improve iterations

- Use function abstractions (such as map, reduce, filter etc.) over
  for-of loops to gain benefits of having less side effects and easier
  readability.
- Enable `downLevelIterations` for writing modern code with lazy evaluation.
- Refactor for of loops to named abstractions to clearly express their
  intentions without needing to analyse the loop itself.
- Add missing cases for changes that had no tests.

tests/unit/stubs/CategoryStub.ts

@@ -27,10 +27,9 @@ export class CategoryStub extends BaseEntity<number> implements ICategory {
   }
 
   public withScriptIds(...scriptIds: string[]): CategoryStub {
-    for (const scriptId of scriptIds) {
-      this.withScript(new ScriptStub(scriptId));
-    }
-    return this;
+    return this.withScripts(
+      ...scriptIds.map((id) => new ScriptStub(id)),
+    );
   }
 
   public withScripts(...scripts: IScript[]): CategoryStub {