New branch_exit_companions= (?) for unnecessary_nesting_linter()

[MichaelChirico]

Consider:

if (A) {
  stop('abc')
} else {
  warning('def')
}

if (B) {
  1 + 1
} else {
  warning("ghi")
}

if (C) {
  2 + 2
} else {
  myStop('fail')
}

We want the flexibility to allow warning() to be considered "parallel" to stop() in the A branch, but not to generate any lint in the B branch (which can't be unnested because there are no actual exit calls).

I think that requires a new parameter, I'm calling it branch_exit_companions= for now. This defines a set of calls that can "match" an exit call without themselves being considered one. Desired behavior:

linter	A	B	C
unnecessary_nesting_linter()	lint 'stop'	no lint	no lint
unnecessary_nesting_linter(branch_exit_calls = 'myStop')	lint 'stop'	no lint	lint 'myStop'
unnecessary_nesting_linter(branch_exit_companions = 'warning')	no lint	no lint	no lint

Current behavior:

lint(linters = list(
  default = unnecessary_nesting_linter(),
  warning_exit = unnecessary_nesting_linter(branch_exit_calls = 'warning')
), text = R"(
if (A) {
  stop('abc')
} else {
  warning('def')
}

if (B) {
  1 + 1
} else {
  warning("ghi")
}

if (C) {
  2 + 2
} else {
  myStop('fail')
}
)")

# <text>:2:1: warning: [default] Reduce the nesting of this if/else statement by unnesting the portion without an exit clause, i.e., stop().
# if (A) {
# ^~~~~~~~
# <text>:8:1: warning: [warning_exit] Reduce the nesting of this if/else statement by unnesting the portion without an exit clause, i.e., warning().
# if (B) {
# ^~~~~~~~