add tidy methods for model_fit objects

[topepo]

That just try to run tidy on the fit element.

[alexpghayes]

Can I suggest an informative error along the lines of "tidy.{class} not available..." when this doesn't work? broom has been trying to move away from "try something that works most of the time" to "only give people results we're sure are right or fail loudly" and it would be nice to mirror this philosophy in tidy() outside of broom.

The trick being which class to list in the error method. I think it should be the _{class}, not class(model_fit) or class(model_fit$fit).

[topepo]

My thoughts were to just run try(tidy(object), silent = TRUE) and give an intelligible error message based on the results. We could (also) look to see if there is a registered tidy methods too (but that might not be foolproof).

[malcolmbarrett]

I'll take a crack at this

[github-actions]

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