Add helper functions for making shared pointers to rules

- start work on item set class

src/lr/item_set.cpp

@@ -0,0 +1,13 @@
+#include "item_set.h"
+
+namespace tree_sitter {
+    namespace lr {
+        ItemSet::ItemSet(const Item &item, const Grammar &grammar) {
+
+        }
+        
+        TransitionMap<ItemSet> ItemSet::transitions() const {
+            return TransitionMap<ItemSet>();
+        }
+    }
+}