Feols: Implement R2 for WLS

[s3alfisc]

Context

Implement support for R2 measures for weighted least squares.

So far, this is not implemented, as there are quite a few options out there. For details on what R's stats package does, see here. Likely fixest follows stats, which we would have to confirm.

To do

• Research how fixest implements R2 for WLS.
• Implement R2 measures for WLS in Feols.get_performance().
• Update summary and etable to return R2 measures if regression weights are employed.
• Implement unit tests against fixest in tests/test_vs_fixest.py::test_single_fit.

[s3alfisc]

Awesome 🎉 Thanks for taking it up!