added stubs (haxe3 transitional)

std/cpp/Utf8.hx

@@ -0,0 +1,5 @@
+package cpp;
+
+#if !haxe3
+typedef Utf8 = haxe.Utf8;
+#end

std/neko/Utf8.hx

@@ -0,0 +1,5 @@
+package neko;
+
+#if !haxe3
+typedef Utf8 = haxe.Utf8;
+#end

std/php/Utf8.hx

@@ -0,0 +1,5 @@
+package php;
+
+#if !haxe3
+typedef Utf8 = haxe.Utf8;
+#end