Node v14: internal/types.d.ts(45,13): Property 'observable' does not exist on type 'SymbolConstructor'.

## Bug Report

**Current Behavior**
Issue #3674 is resurrected with @types/node version 14.

Looks like it was removed in https://github.com/DefinitelyTyped/DefinitelyTyped/pull/40927.

**Environment**
- Runtime: Node v14
- RxJS version: 6.6.3
