Commit details:
- Refactor the Abstract Syntax Tree (AST) node hierarchy in `main.cxx` for improved memory management and ownership semantics.
- Introduce smart pointers (`std::unique_ptr`) for managing ownership of AST nodes, eliminating manual memory management.
- Add getter functions to access operands in Unary and Binary nodes.
- Implemente functions to release ownership of operands in Unary and Binary nodes.
- Replace manual deletion of operands in the destructor with default virtual destructors for Unary and Binary nodes.
- Add a conditional compilation block (`#ifdef ENABLE_TESTS`) to enable testing functionality.
- Create a test expression in the `runTest` function to evaluate a complex mathematical expression.
- Enable test execution using the `--run-test` command-line option.
- Provide a help message for the main program, outlining available options.
