[refactoring] Use of the correct environment to interpret R-Python.

- Split interpreter.rs into expression_eval.rs and statement_execute.rs
- Moved expression evaluation logic into expression_eval.rs
- Moved statement execution logic into statement_execute.rs
- Created mod.rs to expose the main functions (eval, run, execute)
- Removed the old interpreter.rs file
- Fixed unused import warning in statement_execute.rs
- All tests pass successfully

Author: rbonifacio