wald_test should be defined as a standalone function in wald_test.py

[wald_test ](https://github.com/py-econometrics/pyfixest/blob/ef1d0449e5af4b1e697c5edb6d496a1e07d456d5/pyfixest/estimation/feols_.py#L996) should be defined as a standalone function in `wald_test.py`