fix(types): move Symbol.observable into types.ts (#6178)

Closes #6175

src/internal/symbol/observable.ts

@@ -1,11 +1,2 @@
-/** Symbol.observable addition */
-/* Note: This will add Symbol.observable globally for all TypeScript users,
-  however, we are no longer polyfilling Symbol.observable */
-declare global {
-  interface SymbolConstructor {
-    readonly observable: symbol;
-  }
-}
-
 /** Symbol.observable or a string "@@observable". Used for interop */
 export const observable = (() => typeof Symbol === 'function' && Symbol.observable || '@@observable')();

src/internal/types.ts

@@ -1,6 +1,16 @@
 import { Observable } from './Observable';
 import { Subscription } from './Subscription';
 
+/**
+ * Note: This will add Symbol.observable globally for all TypeScript users,
+ * however, we are no longer polyfilling Symbol.observable
+ */
+declare global {
+  interface SymbolConstructor {
+    readonly observable: symbol;
+  }
+}
+
 /** OPERATOR INTERFACES */
 
 export interface UnaryFunction<T, R> { (source: T): R; }