Lint implicit return assignments in function_return_linter()

[IndrajeetPatil]

Follow-up on #1569

Extend function_return_linter() to also lint implicit return assignments like the following (cf. #1569 (comment)):

foo <- function() {
  x <- 4
}
y <- foo() 
y
#> [1] 4

library(lintr)

lint(
  text = "foo <- function() {
            x <- 4
          }",
  linters = function_return_linter()
)

^{Created on 2022-09-30 with reprex v2.0.2}

[MichaelChirico]

Should function_return_linter() be subsumed into return_linter()?

[IndrajeetPatil]

Yes, I think that's a good idea.