Add SceneDependencyCache to Unity gitignore

This is a cache file that should be ignored. Here is a forum post confirming it should be ignored (from Unity employee Joachim_Ante):

https://forum.unity.com/threads/scenedependencycache-shouldnt-be-an-asset.863140/

Unity.gitignore

@@ -69,3 +69,7 @@ crashlytics-build.properties
 # Temporary auto-generated Android Assets
 /[Aa]ssets/[Ss]treamingAssets/aa.meta
 /[Aa]ssets/[Ss]treamingAssets/aa/*
+
+# Cache files for ECS subscenes
+/[Aa]ssets/SceneDependencyCache
+/[Aa]ssets/SceneDependencyCache.meta