feat(furvm): add type function to thing

furvm/include/furvm/thing.hpp

@@ -94,6 +94,13 @@ public:
         }
         return std::move(res);
     }
+public:
+    /**
+     * @brief Returns the thing's type.
+     *
+     * @return The type.
+     */
+    constexpr auto type() const { return m_type; }
 public:
     /**
      * @brief Returns the thing's int32 value.