Tidyselect operations `!` and `-` are subtly different

[lionel-]

Closes https://github.com/tidyverse/dplyr/issues/6959

[dholstius]

Here's an example of a not-so-subtle difference? The final line silently declines to return foo at all.

select(mtcars, foo = cyl)  # result omits unnamed columns
select(mtcars, foo = cyl, !cyl)  # result includes unnamed columns
select(mtcars, foo = cyl, everything(), -cyl)  # result includes all columns except cyl (and foo)