[electron] Can't start the with-electron example.

Hi,

I'm trying to setup expo with electron.

I've followed this : https://docs.expo.io/versions/latest/guides/using-electron/.

But it won't start due to :

```
┏ Renderer -------------------

Error: Cannot find entry file index.ts (or main.ts, or app.ts, or index.js, or main.js, or app.js)
at computeEntryFile (/home/x/Clones/test/node_modules/electron-webpack/src/main.ts:389:11)
at WebpackConfigurator.configure (/home/x/Clones/test/node_modules/electron-webpack/src/main.ts:222:29)
at configure (/home/x/Clones/test/node_modules/electron-webpack/src/main.ts:376:10)

┗ ----------------------------
```

I've tried to start your example, without success.

Even tho I've fresh clone, yarn install and followed this [README.md](https://github.com/expo/examples/tree/master/with-electron).

I know I could workaround it by adding a index.js but from the documentation it's suppose to find the App.js by its own.

Any idea?