Calculates McKelvey and Zavoinas pseudo R2.

## Details

McKelvey and Zavoinas R2 is based on the explained variance,
where the variance of the predicted response is divided by the sum
of the variance of the predicted response and residual variance.
For binomial models, the residual variance is either `pi^2/3`

for logit-link and 1 for probit-link. For poisson-models, the
residual variance is based on log-normal approximation, similar to
the *distribution-specific variance* as described in
`?insight::get_variance`

.