feat(shields): Add ZMK Uno split setup

* Make it easier to test split code with ZMK Uno
  hardware by adding a split left/right setup as well.

app/boards/shields/zmk_uno/zmk_uno_split_left.overlay

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "zmk_uno_split.dtsi"
+
+&left_encoder {
+    status = "okay";
+};