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/NodeMetadataStub.ts

@@ -9,7 +9,7 @@ export class NodeMetadataStub implements NodeMetadata {
 
   public readonly docs: readonly string[] = [];
 
-  public children?: readonly NodeMetadata[] = [];
+  public children: readonly NodeMetadata[] = [];
 
   public readonly type: NodeType = NodeType.Category;