Extract BERT conversion functions into separate module

- Create src/variant_bert.h and src/variant_bert.cpp
- Move bert_to_variant and variant_to_bert functions to new module
- Remove static keyword to allow cross-file usage
- Add guard conditions for null pointer checks
- Fix PROJECTION encoding to use columns[i].x/y/z/w format
- Update godot_cnode.cpp to include variant_bert.h
- Improves code organization and maintainability

Author: fire