Add string rules

src/rules/string.h

@@ -0,0 +1,21 @@
+#ifndef __tree_sitter__string__
+#define __tree_sitter__string__
+
+#include "rule.h"
+
+namespace tree_sitter  {
+    namespace rules {
+        class String : public Rule {
+        public:
+            String(std::string value);
+            TransitionMap<Rule> transitions() const;
+            String * copy() const;
+            bool operator==(const Rule& other) const;
+            std::string to_string() const;
+        private:
+            std::string value;
+        };
+    }
+}
+
+#endif