Sets the main component for Expo to use for your app.
Note: Prior to SDK 18, it was necessary to use registerRootComponent directly, but for projects created as of SDK 18 or later, this is handled automatically in the Expo SDK.
Note:registerRootComponent is roughly equivalent to React Native's AppRegistry.registerComponent, with some additional hooks to provide Expo specific functionality.
You can set the "main" in package.json to any file within your
project. If you do this, then you need to use registerRootComponent;
export default will not make this component the root for the Expo app
if you are using a custom entry file.
For example, let's say you want to make "src/main.js" the entry file
for your app -- maybe you don't like having JavaScript files in the
project root, for example. First, set this in package.json:
{"main":"src/main.js"}
Then in "src/main.js", make sure you call registerRootComponent and
pass in the component you want to render at the root of the app.