Add support for macOS(Darwin) / Windows (#68)

* Update to use`sharedDarwinSource` in iOS

* Renamed `iOS` to `darwin`

* There doesn't seem to be any iOS-specific code. Therefore, by using shared Darwin (Apple) implementations, macOS can be easily supported.

* Add support for macOS

* Support for FlutterMacOS in Darwin

* Fixed attempt to load `libwebcrypto.so` on macOS

* Add macOS to example

* Add support for Windows

* Add windows plugin file

* Add windows library path to `lookup.dart`

Author: CoreNion

ios/.gitignore renamed to darwin/.gitignore

@@ -14,17 +14,36 @@
  * limitations under the License.
  */
 
+#if os(iOS)
 import Flutter
-import UIKit
+#elseif os(macOS)
+import FlutterMacOS
+#endif
+
+import Foundation
 
 public class SwiftWebcryptoPlugin: NSObject, FlutterPlugin {
   public static func register(with registrar: FlutterPluginRegistrar) {
-    let channel = FlutterMethodChannel(name: "webcrypto", binaryMessenger: registrar.messenger())
+    #if os(iOS)
+      let messenger = registrar.messenger()
+    #else
+      let messenger = registrar.messenger
+    #endif
+
+    let channel = FlutterMethodChannel(name: "webcrypto", binaryMessenger: messenger)
     let instance = SwiftWebcryptoPlugin()
     registrar.addMethodCallDelegate(instance, channel: channel)
   }
 
   public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
-    result("iOS " + UIDevice.current.systemVersion)
+    #if os(iOS)
+      let platform = "iOS "
+    #elseif os(macOS)
+      let platform = "macOS "
+    #else
+      let platform = "unknown "
+    #endif
+
+    result(platform + ProcessInfo.processInfo.operatingSystemVersionString)
   }
 }

ios/Assets/.gitkeep renamed to darwin/Assets/.gitkeep

@@ -14,7 +14,13 @@
  * limitations under the License.
  */
 
-#import <Flutter/Flutter.h>
+#include <TargetConditionals.h>
+
+#if TARGET_OS_IOS
+  #import <Flutter/Flutter.h>
+#elif TARGET_OS_OSX
+  #import <FlutterMacOS/FlutterMacOS.h>
+#endif
 
 @interface WebcryptoPlugin : NSObject<FlutterPlugin>
 @end