Improve `Observable` typing in TS and Flow definitions

[novemberborn]

We currently type Observable like so:

export interface ObservableLike {
	subscribe(observer: (value: any) => void): void;
}

I think we should include a more complete type definition. See the index.d.ts and index.js.flow files.

[novemberborn]

This has caused an issue in the past: #1236.

[sindresorhus]

@novemberborn How about this one?

[novemberborn]

@sindresorhus I don't use observables much, so sure 😄

[novemberborn]

@sindresorhus I'm not sure we fully understood this change. #1236 referred to a Flowtype issue. For TypeScript, before this change, anything with a subscribe() would've been a valid test implementation return value, but now it also needs the [Symbol.observable]() function. Do we want to require that second property? Should we support either property instead?

[sindresorhus]

I feel that "just a subscribe function" is too loose. It could accidentally match just a normal object too. The TS type now ensures it's an actual Observable. All the Observable libraries I know of uses Symbol.observable. We can reconsider if it causes any problems.

[micaelmbagira]

@sindresorhus @novemberborn I believe this fix broke something else. The following test in TypeScript:

import { of } from 'rxjs';

test('foo', t => {
  return of(3).pipe(tap(() => t.pass()));
});

does not compile because of the following error

error TS2345: Argument of type '"foo"' is not assignable to parameter of type 'OneOrMoreMacros<[(t: ExecutionContext<{}>) => Observable], {}>'.

test('foo', t => {
~~~~~

Removing the [Symbol.observable](): ObservableLike; solves it though....

When trying to enforce the ObservableLike type with return of(3).pipe(tap(() => t.pass())) as ObservableLike;. TypeScript would throw

Conversion of type 'Observable' to type 'ObservableLike' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
Property '[Symbol.observable]' is missing in type 'Observable' but required in type 'ObservableLike'

So basically, [Symbol.observable]() is not in rxjs Observable type definition. Can we then reopen this issue?