Properly handle lambda function bodies in `unnecessary_nesting_linter()`

[salim-b]

Compare

code <- "foo <- function(x) {
  x + 1
}"
writeLines(code)
#> foo <- function(x) {
#>   x + 1
#> }
lintr::lint(
    text = code,
    linters = lintr::unnecessary_nesting_linter()
)
#> ℹ No lints found.

^{Created on 2025-02-13 with reprex v2.1.1}

vs.

code <- "foo <- \\(x) {
  x + 1
}"
writeLines(code)
#> foo <- \(x) {
#>   x + 1
#> }
lintr::lint(
    text = code,
    linters = lintr::unnecessary_nesting_linter()
)
#> <text>:1:13: warning: [unnecessary_nesting_linter] Reduce the nesting of this statement by removing the braces {}.
#> foo <- \(x) {
#>             ^

^{Created on 2025-02-13 with reprex v2.1.1}

Linting this case should be the responsibility of #2240, instead.