Make Unity3dAsyncAwaitUtil usable in Editor

As per modesttree/Unity3dAsyncAwaitUtil/modesttree#9, using Unity3dAsyncAwaitUtil in
Editor scripts causes an NPE, because `SyncContextUtil.Install` is never called
to initialize global variables `SyncContextUtil.UnitySynchronizationContext` and
`SyncContextUtil.UnityThreadId`.

Fix this by adding the `[InitializeOnLoadMethod()]` attribute to the
`SyncContextUtil.Install` method. Note that the existing
`[RuntimeInitializeOnLoadMethod]` attribute is still needed in order for
`SyncContextUtil.Install` to be invoked when entering Play Mode.

Fixes modesttree/Unity3dAsyncAwaitUtil/modesttree#9

UnityProject/Assets/Plugins/AsyncAwaitUtil/Source/Internal/SyncContextUtil.cs

@@ -6,10 +6,17 @@
 using System.Threading.Tasks;
 using UnityEngine;
 
+#if UNITY_EDITOR
+using UnityEditor;
+#endif
+
 namespace UnityAsyncAwaitUtil
 {
     public static class SyncContextUtil
     {
+#if UNITY_EDITOR
+        [InitializeOnLoadMethod()]
+#endif
         [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
         static void Install()
         {