feat(macOS): Make podspec compatible with macOS (#1328)
* [apple] Move iOS/macOS src into common apple dir
* [apple] Enable macOS as platform in podspec
* [example] Use CocoaPods & auto-linking on macOS
* [docs] Update setup for macOS
* [package] Include apple dir in distribution
Hello, in order to use react-native-webview > 6.0.2 please make your android/gradle.properties contains:
```
android.useAndroidX=true
android.enableJetifier=true
```
This enables AndroidX libraries which are the new standard.
fixes #580
fixes #581
fixes #582