Preserve C# 4 compatibility

Unity's C# compiler can't handle declaration expressions[1], so we need to do it in a semantically identical but syntactically different way.

[1] A declaration expression is something like:

DoSomething(out var foo);

instead of

string foo;
DoSomething(out foo);

YarnSpinner/AntlrCompiler.cs

@@ -817,7 +817,8 @@ public String toDot()
 			buf.Append("\n");
 			foreach (String src in edges.Keys)
 			{
-				if (edges.TryGetValue(src, out var output))
+                IList<string> output;
+				if (edges.TryGetValue(src, out output))
 				{
 					foreach (String trg in output)
 					{