Add new rule to warn about async functions without await

[ajafff]

This rule requires at least one await expression in the function body.
@AlCalzone suggested such a rule in microsoft/TypeScript#22024

This rule would not advertise fixes, because async functions automagically catch sync exceptions and convert them to rejected promises. Removing the async modifier during fixing could change the runtime behavior of the function.

[ajafff]

The implementation is not the hard part. But this rule needs a good name.
ESLint has require-await, but one could think it's somehow about require().

[ajafff]

Because of the implicit catching of sync exceptions in async functions, there's actually no way to detect if the async modifier is unnecessary. Ergo this rule does not prevent any programmer error but will lead to a lot of confusion as the runtime behavior changes.
That said and given the arguments in eslint/eslint#10000 I'm not convinced this rule needs to be part of the core library.