feat(config): add default_workspace_layout opt

This commit adds a default_workspace_layout opt, which defaults to BSP
to maintain backwards compatibility. This can also be set to "None".

When set to "None" or omitted, the default behaviour for new or
undefined workspaces (i.e. on monitors without config blocks) will be
non-tiling.  Otherwise, the given value will be the default layout
applied.

schema.json

@@ -152,6 +152,18 @@
       "format": "int32",
       "default": 10
     },
+    "default_workspace_layout": {
+      "description": "Global default workspace layout for new workspaces",
+      "anyOf": [
+        {
+          "$ref": "#/$defs/DefaultLayout"
+        },
+        {
+          "type": "null"
+        }
+      ],
+      "default": "BSP"
+    },
     "default_workspace_padding": {
       "description": "Global default workspace padding",
       "type": [