Add Expo support

[zihejia]

This PR introduces support for Expo, React Native Web, and any platform using React Native that does not support iOS and Android. To activate this feature, initialize Mixpanel with an additional parameter useNative set to false. This will enable Javascript mode.

 const trackAutomaticEvents = false;
 const useNative = false;
 const mixpanel = new Mixpanel(
    "YOUR_MIXPANEL_TOKEN",
    trackAutomaticEvents,
    useNative
  );
 mixpanel.init();

To try the Expo sample app, navigate to Samples/MixpanelExpo, run npm install, and then execute npm run ios or npm run android

Known limitations and differences compared to the native mode (iOS/Android):

• Automatic Events are currently not supported in this mode. Setting 'trackAutomaticEvents' to 'true' will have no effect.
• Certain Mixpanel Properties are unavailable in Javascript mode, including detailed information about the device and screen.
• The default flush interval is set to 10 seconds. The data will not flush automatically when the app moves to the background. We recommend flushing more frequently for key events.