unawaited_futures doesn't detect unawaited futures in non-async functions

[DetachHead]

Describe the issue
if a Future is not awaited within a non-async function, the unawaited_futures warning is not triggered

To Reproduce

Future<void> foo() => throw UnimplementedError();

void syncFunction() {
  foo(); //no warning
}

Future<void> asyncFunction() async {
  foo(); //warning: `Future` results in `async` function bodies must be `await`ed or marked `unawaited` using `dart:async`
}

Expected behavior
warning when not awaiting a Future even when the function is not async, as it's just as likely to be a mistake either way

[pq]

This has come up a bunch.

See also: #58512