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

react-native-webview.podspec

@@ -10,10 +10,10 @@ Pod::Spec.new do |s|
 
   s.authors      = package['author']
   s.homepage     = package['homepage']
-  s.platform     = :ios, "9.0"
+  s.platforms    = { :ios => "9.0", :osx => "10.14" }
 
   s.source       = { :git => "https://github.com/react-native-community/react-native-webview.git", :tag => "v#{s.version}" }
-  s.source_files  = "ios/**/*.{h,m}"
+  s.source_files  = "apple/**/*.{h,m}"
 
   s.dependency 'React'
 end