feat(Windows): Adds Windows support to react-native-video

This PR adds react-native-windows support to react-native-video.  The Video component is implemented with a XAML MediaPlayerElement.  Most of the features implemented by Android (and some additional ones) are implemented by Windows.

Known issues and missing features include:
* onReadyForDisplay event
* local URI convention (e.g., "broadchurch" in examples changed to require("./broadchurch.mp4")
* `playableDuration` in `onVideoProgress` event is always 0.0
* `playInBackground` is not yet supported
* Volume settings are applied, but the UWP control does not handle it properly

windows/ReactNativeVideo/ReactVideoEventTypeExtensions.cs

@@ -0,0 +1,36 @@
+using System;
+using static System.FormattableString;
+
+namespace ReactNativeVideo
+{
+    static class ReactVideoEventTypeExtensions
+    {
+        public static string GetEventName(this ReactVideoEventType eventType)
+        {
+            switch (eventType)
+            {
+                case ReactVideoEventType.LoadStart:
+                    return "onVideoLoadStart";
+                case ReactVideoEventType.Load:
+                    return "onVideoLoad";
+                case ReactVideoEventType.Error:
+                    return "onVideoError";
+                case ReactVideoEventType.Progress:
+                    return "onVideoProgress";
+                case ReactVideoEventType.Seek:
+                    return "onVideoSeek";
+                case ReactVideoEventType.End:
+                    return "onVideoEnd";
+                case ReactVideoEventType.Stalled:
+                    return "onPlaybackStalled";
+                case ReactVideoEventType.Resume:
+                    return "onPlaybackResume";
+                case ReactVideoEventType.ReadyForDisplay:
+                    return "onReadyForDisplay";
+                default:
+                    throw new NotSupportedException(
+                        Invariant($"No event name added for event type '{eventType}'."));
+            }
+        }
+    }
+}