Helper function to obtain fixed effect and grouping factor?

Given that `reformulas` takes formulas of random-effects components and processes them, I'm surprised there is not an explicit function to obtain fixed effects and grouping factors (only). I tested a little bit to get something to work, but I feel like this should already be built in...

```
formula <- angle ~ recipe * temperature + (1|recipe:replicate)

# getting grouping factor
group_vars <- reformulas::findbars(formula)
group_vars <- unique(
unlist(lapply(group_vars, function(term) all.vars(term[[3]]))))
group_vars
```
```
[1] "recipe" "replicate"
```

```
# getting fixed effects (only)
bars <- reformulas::RHSForm(formula)
fixed_vars <- all.vars(bars)
# trying to get rid of possible grouping factors (that appear in both areas)
fixed_vars <- setdiff(fixed_vars, group_vars)
fixed_vars
```
```
[1] "temperature"
```

Of course, do let me know if there's in fact an easier way and I might have missed something...

Start by reading the existing reformulas::findbars and RHSForm entry points and tracing them with the example formula in the issue. Clarify the intended helper's return values for fixed effects and grouping factors, then verify that the documented example produces the expected separation.

Written by the indexing model from the issue text.