fix(types): Add a missing import for Symbol.observable

This was reported previously on ReactiveX#3674, but instead of adding
this import, it was fixed in @types/node: DefinitelyTyped/DefinitelyTyped#26034.

This was removed in v14 of @types/node.

Fix it properly this time.

Fixes ReactiveX#5861

src/internal/types.ts

@@ -1,5 +1,6 @@
 import { Observable } from './Observable';
 import { Subscription } from './Subscription';
+import './symbol/observable';
 
 /** OPERATOR INTERFACES */