Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

src/domain/ICategoryCollection.ts

@@ -12,8 +12,8 @@ export interface ICategoryCollection {
   readonly actions: ReadonlyArray<ICategory>;
 
   getScriptsByLevel(level: RecommendationLevel): ReadonlyArray<IScript>;
-  findCategory(categoryId: number): ICategory | undefined;
-  findScript(scriptId: string): IScript | undefined;
+  getCategory(categoryId: number): ICategory;
+  getScript(scriptId: string): IScript;
   getAllScripts(): ReadonlyArray<IScript>;
   getAllCategories(): ReadonlyArray<ICategory>;
 }