`posteriordraws()` -> `get_draws()`

[vincentarelbundock]

Can also be used to extract bootstrap and simulation-based inference draws. Would also fit in nicely with a bunch of helper functions that marginaleffects offers: get_vcov(), get_coef(), etc.

posteriordraws() would remain as an alias forever (without warning or deprecation or anything), since it is used in the JSS article.