feat(config): add monitor index prefs to static

This commit adds an "monitor_index_preferences" key to the static config
schema, which was missed during the initial rollout of the static
configuration files. To help with testing, these indexes have also been
exposed on the State struct.

resolve #522

schema.json

@@ -154,6 +154,16 @@
         "$ref": "#/definitions/IdWithIdentifier"
       }
     },
+    "monitor_index_preferences": {
+      "description": "Set monitor index preferences",
+      "type": [
+        "object",
+        "null"
+      ],
+      "additionalProperties": {
+        "$ref": "#/definitions/Rect"
+      }
+    },
     "monitors": {
       "description": "Monitor and workspace configurations",
       "type": [