fix(TypeScript): Symbol.observable is now unique symbol.

Resolves #41

BREAKING CHANGE: Older versions of `@types/node` will clash with this, because `@types/node` incorrectly defined `Symbol.observable` has `symbol` and not `unique symbol`. Update to `@types/node@14.14.6` or higher.

index.d.ts

@@ -3,6 +3,6 @@ export default observableSymbol;
 
 declare global {
   export interface SymbolConstructor {
-    readonly observable: symbol;
+    readonly observable: unique symbol;
   }
 }