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.

src/application/Parser/CategoryParser.ts

@@ -9,11 +9,6 @@ import { parseScript } from './Script/ScriptParser';
 
 let categoryIdCounter = 0;
 
-interface ICategoryChildren {
-  subCategories: Category[];
-  subScripts: Script[];
-}
-
 export function parseCategory(
   category: CategoryData,
   context: ICategoryCollectionParseContext,
@@ -48,6 +43,11 @@ function ensureValid(category: CategoryData) {
   }
 }
 
+interface ICategoryChildren {
+  subCategories: Category[];
+  subScripts: Script[];
+}
+
 function parseCategoryChild(
   data: CategoryOrScriptData,
   children: ICategoryChildren,