[suffix-subjects] Subjects with dollar sign ending don't cause an error

[JasonWeinzierl]

The suffix-subjects rule isn't effecting a failure for the following code:

import { Subject } from 'rxjs';

const sourceSubject$ = new Subject<number>();
// Should cause an error because the suffix should be Subject, not Subject$.

It's possible this is intentional, to be compatible with finnish, but I can't tell for sure looking over the history of this rule.

Possibly relevant history:

• Initial implementation: cartant@e185ed8
• Bug fixes for $$ suffix: [Identify subjects] Doesn't work with BehaviorSubject, EtcSubject  cartant/eslint-plugin-rxjs#88
• 4.0.3 change: cartant@a23a69c
• 5.0.2 change: cartant@d4a94c0

[JasonWeinzierl]

After taking a closer look at the unit tests (and the older tslint implementation cartant/rxjs-tslint-rules#77), it looks like this is intentional behavior. Instead of changing behavior, we will explicitly document this behavior in the suffix-subjects documentation. See #91 for documentation changes.