[jss] Use indefinite-observable for Observable typings

The ones in HEAD are copy-pasted from there.  Lets link to the original source.

Continuation of #24078

types/jss/css.d.ts

@@ -1,6 +1,6 @@
 // These CSS typings adapted from TypeStyle: https://github.com/typestyle/typestyle
 
-import { Observable } from './observable';
+import { Observable } from 'indefinite-observable';
 import * as csstype from 'csstype';
 
 type Length = string | number;

types/jss/observable.d.ts

@@ -1,17 +0,0 @@
-// Copied from https://github.com/cssinjs/jss/blob/6ed7963786d5ef899075e95f81efc9530342154f/src/types.js
-
-export type Observable<T> = {
-	subscribe(observerOrNext: ObserverOrNext<T>): Subscription
-}
-
-export type Observer<T> = {
-	next: NextChannel<T>
-}
-
-export type NextChannel<T> = (value: T) => void
-export type ObserverOrNext<T> = Observer<T> | NextChannel<T>
-
-export type Unsubscribe = () => void
-export type Subscription = {
-	unsubscribe: Unsubscribe
-}

types/jss/package.json

@@ -1,6 +1,7 @@
 {
   "private": true,
   "dependencies": {
-    "csstype": "^2.0.0"
+    "csstype": "^2.0.0",
+    "indefinite-observable": "^1.0.1"
   }
 }