summ method not working with mixed models

**Is your feature request related to a problem? Please describe.**

I am not able to use summ method with fitted objects from the glmmTMB mixed models,

**Describe the solution you'd like**

**Describe any alternatives or other implementations that you might know of**

**Additional context**

Start by reproducing the reported failure with the `summ` method and a fitted `glmmTMB` mixed-model object. Then trace the `summ` entry point and compare its supported fitted-object types. Done means `summ` works with `glmmTMB` fitted objects, with coverage for the reported case.

Written by the indexing model from the issue text.