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.conf

@@ -0,0 +1,18 @@
+# Copyright (c) 2022 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+CONFIG_LOG=y
+CONFIG_ZMK_LOG_LEVEL_DBG=y
+
+# Uncomment for Single color backlight
+# CONFIG_ZMK_BACKLIGHT=y
+
+# Uncomment for RGB
+# CONFIG_ZMK_RGB_UNDERGLOW=y
+
+# Uncomment for Display
+# CONFIG_ZMK_DISPLAY=y
+
+# Uncomment these two lines to enable encoder support
+# CONFIG_EC11=y
+# CONFIG_EC11_TRIGGER_GLOBAL_THREAD=y