Support factor predictors with continuous moderators in sim_slopes/johnson_neyman

This would involve finding a j_n interval at each level of the factor predictor. For now, would involve too much of a rework of the underlying functions for me to do it. It's not too much unlike what is done for second moderators, though.

Start with the sim_slopes and johnson_neyman entry points, then compare how second moderators are handled. Determine the underlying rework needed to compute a Johnson–Neyman interval at each level of a factor predictor; the work is done when both functions support those intervals consistently.

Written by the indexing model from the issue text.