Prep for eas build

[bpringe]

These small changes make it so that this project will work out of the box with EAS Build. I left out the EAS Build configuration because that is somewhat account-specific.

With EAS Build, if no App.js or index.js file exists at the root, it creates and index.js file that tries to import ./App, which it expects to export a root component, which it passes to registerRootComponent. This causes an error during the build process that looks like:

[stderr] Error loading assets JSON from Metro. Ensure you've followed all expo-updates installation steps correctly. Unable to resolve module ./App from /build/workingdir/build/index.js: 
[stderr] None of these files exist:
[stderr]   * App(.native|.android.ts|.native.ts|.ts|.android.tsx|.native.tsx|.tsx|.android.js|.native.js|.js|.android.jsx|.native.jsx|.jsx|.android.json|.native.json|.json)
[stderr]   * App/index(.native|.android.ts|.native.ts|.ts|.android.tsx|.native.tsx|.tsx|.android.js|.native.js|.js|.android.jsx|.native.jsx|.jsx|.android.json|.native.json|.json)
[stderr]   2 | import { registerRootComponent } from 'expo';
[stderr]   3 |
[stderr] > 4 | import App from './App';
[stderr]     |                  ^
[stderr]   5 |
[stderr]   6 | // registerRootComponent calls AppRegistry.registerComponent('main', () => App);
[stderr]   7 | // It also ensures that whether you load the app in Expo Go or in a native build,

Since we're calling registerRootComponent in our ClojureScript, we want EAS Build to load our ./app/index.js file instead of creating one and loading that. To do this, we create our own index.js file at the root and import ./app/index.js.

Since the eas-cli by default does not send files to the build server that are ignored in .gitignore, and we want to ignore the shadow compilation output files, we use a build-specific npm hook that EAS Build will call to compile the cljs during the build process. The eas-build-post-install script in package.json is used to do this.

I've tested these changes myself in another branch on my fork by configuring EAS Build and running a build that creates an apk, downloading that apk, and running it on an emulator.

[PEZ]

Thank you! This is awesome. I have tested that it works with non-EAS builds. Check!