[unicorne] Add a layout alias (#23056)

Add a `LAYOUT` alias to support copying of keymaps from crkbd

keyboards/boardsource/unicorne/info.json

@@ -119,6 +119,9 @@
         "sleep": true
     },
     "community_layouts": ["split_3x6_3"],
+    "layout_aliases": {
+        "LAYOUT": "LAYOUT_split_3x6_3"
+    },
     "layouts": {
         "LAYOUT_split_3x6_3": {
             "layout": [