load subtree variable definition

sample/vars/test-subtree.json

@@ -4,8 +4,15 @@
   "firstid": 1,
   "export": true,
   "declare": {
-    "imports": [],
-    "vars": []
+    "imports": [
+      "vars/declare-core.json"
+    ],
+    "vars": [
+      {
+        "name": "test",
+        "desc": "测试"
+      }
+    ]
   },
   "root": {
     "id": 1,