Refine resid() output

[s3alfisc]

Currently, the resid() method will return weighted residuals for WLS estimation.

For compatibility with fixest:::fixest.resid(), we should be default return the unweighted residuals.

Additionally, create a custom resid() method for fepois with function arguments type = "response" (default) or "working", which returns either the unweighted residuals or the irls_weights.