Make App use Main from the new local package

Now webapp will tsc correctly (because we have index.ts at mymain root).
But when `yarn start` it'll fail in browser because mymain not built.
We have to tsc in mymain to build the package and it'l run successfully.

webapp/src/App.tsx

@@ -1,3 +1,4 @@
+import { Main } from 'mymain';
 import * as React from 'react';
 import './App.css';
 
@@ -11,9 +12,7 @@ class App extends React.Component {
           <img src={logo} className="App-logo" alt="logo" />
           <h1 className="App-title">Welcome to React</h1>
         </header>
-        <p className="App-intro">
-          To get started, edit <code>src/App.tsx</code> and save to reload.
-        </p>
+        <Main />
       </div>
     );
   }