feat: add cross-engine runtime support

Runtime/ShrinkContextDefaults.cs

@@ -1,5 +1,7 @@
 #nullable enable
+#if UNITY_5_3_OR_NEWER
 using UnityEngine;
+#endif
 
 namespace ShrinkContext
 {
@@ -13,7 +15,9 @@ namespace ShrinkContext
 
         public static ShrinkContextRuntime Default => _default ??= new ShrinkContextRuntime();
 
+#if UNITY_5_3_OR_NEWER
         [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)]
+#endif
         private static void ResetStaticState()
         {
             _default = null;