promise/catch-or-return should report unhandled Promise.resolve/reject

[epoberezkin]

This innocently looking assignment:

const FAIL = Promise.reject();

causes UnhandledPromiseRejectionWarning if FAIL is not used...

[macklinu]

I believe an unhandled Promise.resolve() is valid, but opening up a Node 8 REPL shows the following:

❯ node
> const FAIL = Promise.reject()
undefined
> (node:99089) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): undefined
(node:99089) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

I think warning for usages of Promise.reject() that aren't returned in a function would be a good addition. For example, assigning Promise.reject() to a variable in your example would be invalid.

I'm not 100% sure if it belongs in the promise/catch-or-return rule or if it should be part of another rule. I think that is worth discussing a little bit.

Discussion and PRs are welcome (even to discuss an initial implementation - we can always move it into another rule at the PR stage).