Add classes to `wt_()` returns

[malcolmbarrett]

important weights class from tidymodels + a class to indicate the type of weight, e.g. ato_weight, which can help us indicate which sandwich estimator to fit

cc @simonpcouch because this idea came from your code. In other words, we can move the step of indicating this is a weight we can use in tidymodels to propensity

[simonpcouch]

Stoked to hear you're motivated to tighten that integration. :)

Wanted to note that our current drafts of machinery introducing support for propensity take care of that conversion under the hood.

[malcolmbarrett]

Cool, I think that will work a bit better since the importance weights class object turned out to be difficult to work with in other ways we use them