New argument for implicit_assignment_linter to skip "scoped" usages

[MichaelChirico]

I sometimes find the following type of usage to be appropriate for "implicit" assignment:

if (any(idx <- x > 0)) {
  stop("Found positive entries: [", toString(which(idx)), "]")
}

The object idx is not used outside the branch, so I think it can be distracting to define it in the "main" body of the function. I believe @IndrajeetPatil has a cite on this being good practice in some contexts too.

It would be nice to allow this sort of implicit assignment, defined by the object of assignment is (1) inside if and (2) only referenced inside that if's parent <expr>.

This argument (let's say allow_scoped_if_assignment) would be FALSE by default since the style guide does not consider this exception.

[MichaelChirico]

One question is whether foo(x <- 1) should be linted if x is not referenced anywhere else in the code. I think yes (esp. since this is a common mistake where = was intended), but it will make implementation a bit harder.