This PR is mainly intended to fix #51, but also includes some additional cleanup.

The error in #51 is caused by platform specific iOS code that doesn't work in the Play mode of the Unity Editor.
This can easily be fixed by checking for NOT editor.

-#if UNITY_IOS
+#if UNITY_IOS && !UNITY_EDITOR
            // Call an obj-C function name
            FlutterEmbedUnityIos_sendToFlutter(data);
#endif

While updating this, I noticed Android uses a runtime check instead of compile-time.
So I combined both checks into #if and #elif logic.

In short:

• Fixes #if UNITY_IOS check by checking for UNITY_EDITOR first.
• Replace Application.platform == RuntimePlatform.Android with #if UNITY_ANDROID for efficiency.
• Debug.Log when SendToFlutter.Send() is called in the Unity Editor, to show the function does something.

The changes are identical for Unity 2022_3 and 6000_0