Return emmeans for interaction terms

Currently `ard_emmeans_mean_difference()` / `ard_emmeans_contrast()` is restricted to primary covariates that exist within the input data?
Could it expand to interaction terms as well?
For example:

```
cardx::ard_emmeans_mean_difference(
  data = mtcars,
  formula = mpg ~ am + cyl + am*cyl,
  method = "lm",
  primary_covariate = "am*cyl"
)
```