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.
- Use better error messages with more context.
- Unify their validation logic and share tests.
- Validate also type of the name.
- Refactor node (Script/Category) parser tests for easier future
changes and cleaner test code (using `TestBuilder` to do dirty work in
unified way).
- Add more tests. Custom `Error` properties are compared manually due to
`chai` not supporting deep equality checks (chaijs/chai#1065,
chaijs/chai#1405).
