fix: use window.setTimeout instead of setTimeout

Node.js' setTimeout returns entire Timer object and after upgrade
the TypeScript considered setTimeout returns NodeJS.Timer object
instead of a number. Changing to use window.setTimeout to distinguish
with Node.js.
See more details: microsoft/TypeScript#842

src/App.tsx

@@ -21,7 +21,7 @@ export default class extends React.Component<any, HelloState> {
     let {i} = this.state;
     i++;
     this.setState({i});
-    this.tickToken = setTimeout(this.tick, 800);
+    this.tickToken = window.setTimeout(this.tick, 800);
   };
 
   render() {