assignment_linter does not fully exclude implicit assignments

```r
lint(text = "if (foo(a<-1)) TRUE", linters = assignment_linter(operator = "="))
# <text>:1:10: style: [assignment_linter] Use = for assignment, not <-.
# if (foo(a<-1)) TRUE
#          ^~
```

Linting this should be the purview of `implicit_assignment_linter()`.