Consider renaming the `IV`, `DV`, and `M` args

[cedricbatailler]

The arguments names might be a bit obscure for people unfamiliar with psych methods idioma.

More appropriate arguments could be:
DV: outcome
IV: predictor
M: mediator
Mod: moderator

If so, we should depreciate the old args. See https://r-pkgs.org/release.html#compatibility