Move logic from `conf_int` method to `confidence_intervals.py`

[s3alfisc]

Logic in the confint method should be moved to confidence_intervals.py

[Dpananos]

I can pick this up.

Any more info beyond what is in the title?

[s3alfisc]

This is a very simple PR - basically I would like to move some logic from the Feols class to standalone scripts. So the idea here is to move everything related to confidence intervals to a separate script.