Introduce `unnecessary_nested_if_linter()`

[IndrajeetPatil]

In the wild (e.g.), I have seen this pattern multiple times: there are nested if() statements, when a single conditional with the right conditional expression would do.

For example, this

x <- TRUE
y <- TRUE

# actual
if (x) {
  if (y) {
    print(1.1)
  }
}
#> [1] 1.1

can be rewritten as:

if (x && y) print(1.1)
#> [1] 1.1

^{Created on 2022-11-25 with reprex v2.0.2}

The benefit is, of course, reduction in complexity and improved readability.

Currently, this doesn't produce a lint:

library(lintr)

code <- "if (x) {
  if (y) {
    print(1.1)
  }
}"

lint(
  text = code,
  linters = linters_with_tags(NULL)
)

^{Created on 2022-11-25 with reprex v2.0.2}

---

P.S. Needless to say, this is relevant only if there isn't some additional code in the outer scope. E.g.

if (x) {
  y <- x + y
  if (y) {
    ...
  }
}

[MichaelChirico]

Here's another one to be wary of edge cases, but again I can't seem to recall why :)