`predictions()`: `variables` argument does not work for factors

[vincentarelbundock]

pre-formula and in-formula both break:

library(marginaleffects)
mod <- lm(mpg ~ factor(vs), data = mtcars)
predictions(mod, variables = list(vs = 0:1))
predictions(mod, variables = list(vs = "pairwise"))

dat <- mtcars
dat$vs <- factor(dat$vs)
mod <- lm(mpg ~ vs, data = dat)
predictions(mod, variables = list(vs = 0:1))
predictions(mod, variables = list(vs = c("0", "1")))
predictions(mod, variables = list(vs = "pairwise"))