Rule proposal: require async for Promise returning methods.

[AndyMoreland]

I'd like to have a lint rule that requires the async modifier be attached to any method that returns a Promise.

Reasoning:

foo(): Promise<T> can (technically) either return a rejected promise or bubble a standard synchronous Error object. No sane API is designed to use both error paths, because it requires any consumer use two separate error handling constructs at any exception handling point.

In my codebase I'd like to require that any method that returns a Promise also be marked async, because async methods automatically catch synchronous exceptions and convert them into rejected promises. This would prevent accidental synchronous exceptions in async codepaths.

[andy-hanson]

Should be fixed by #1779.