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.

tsconfig.json

@@ -7,6 +7,7 @@
     "downlevelIteration": true,
     "jsx": "preserve",
     "moduleResolution": "Node",
+    "strictNullChecks": true,
     "experimentalDecorators": true,
     "esModuleInterop": true,
     "resolveJsonModule": true,