Check dots empty

[dsweber2]

Closes #276. the different commits identify confidence in actually using check_dots_used. The slather cases were the most ambiguous to me

[dajmcdon]

Thanks for doing this. The dots arguments are a bit complicated. See for example: https://adv-r.hadley.nz/s3.html#s3-arguments

My understanding is that

1. The S3 generics in this package need ... because methods may have different arguments. So generics should never check_dots_*.
2. methods "can" have check_dots_*. However, I've tried to make my versions of functions in {recipes} have similar constructions. So, e.g., epi_recipe methods don't check because recipe methods don't check. I'm not sure I could articulate a reason, but my assumption is "they know better".
3. add_model() should check because the {workflows} analogues (e.g., add_preprocessor() does.
4. The slather() methods are more complicated. These aren't particularly user facing, so it may not matter. The bake() methods in {recipes} generally don't check, so I haven't (as in point 2) for example, bake.*()/prep.*()

I've tried to comment on each individually.