Analog to `coef()` for mixed effects models

[bwiernik]

It would be nice to have an analog to coef() for mixed effects models. This returns fixef() + ranef() for each random parameter. Modeling packages vary in whether SEs and intervals are provided for these results (e.g., brms does, glmmTMB hopes to, lme4 won't).

[strengejacke]

We could return those probably using the effects and component arguments, what would you suggest?

[bwiernik]

I think effects = "group_specific" would be good.

[DominiqueMakowski]

"group_level" ?

[bwiernik]

"Level" usually means the specific levels of the group factor though, and there's some confusion with the group_level argument. See also this response https://twitter.com/smartin2018/status/1390738366056960003?s=21