Add optional GDScript linting setup for improved code quality

The project currently does not include a standardized linting setup for GDScript, which can result in inconsistent formatting and reduced readability across contributions.

As the codebase grows and more contributors get involved, having a lightweight and optional linting mechanism can significantly improve maintainability.

Problem

- No consistent GDScript style enforcement
- Harder to review PRs due to formatting differences
- Contributors may unknowingly introduce style issues