Maybe don't allow for type = flexibility

[DavisVaughan]

It would greatly simplify some things, and make it straightforward to add a run_model_matrix = FALSE arg to the formula method of mold() if we always returned a tibble.

It would also generally clean up the mold() fn call. And things would generally be more type stable for the developer, at the cost of some performance loss if the user passes in a matrix to mold(x = <matrix>) and then the developer wants that back as a matrix. (Then again, they wouldn't have to use mold() if they didn't want to, and we could export the add_intercept_column() fn if we wanted to)

[github-actions]

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