Support sdmTMB

[bwiernik]

From @sschooler

It would be amazing if the performance package could start implementing support for sdmTMB. I think @seananderson may have this in the works already, but the last progress appears to be from 2022. Here's the request thread over at SDMtmb GitHub.

[strengejacke]

As reference: https://github.com/pbs-assess/sdmTMB/blob/main/scratch/r2.R
Need to find out how to "reconstruct" an object that is similar to what VarCorr() returns.