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.

tests/unit/shared/Stubs/UseCollectionStateStub.ts

@@ -71,7 +71,7 @@ export class UseCollectionStateStub
     if (scenario.immediateOnly) {
       handlers = handlers.filter((args) => args[1]?.immediate === true);
     }
-    const callbacks = handlers.map((args) => args[0] as NewStateEventHandler);
+    const callbacks = handlers.map((args) => args[0]);
     if (!callbacks.length) {
       throw new Error('No handler callbacks are registered to handle state change');
     }