feat: add _merge property to hugo outputs with mergeType reference (SchemaStore#5219)

Author: Lruihao

src/schemas/json/hugo.json

@@ -3264,6 +3264,11 @@
         "section": ["html", "rss"],
         "taxonomy": ["html", "rss"],
         "term": ["html", "rss"]
+      },
+      "properties": {
+        "_merge": {
+          "$ref": "#/definitions/mergeType"
+        }
       }
     },
     "page": {