highlights(cpp): add support for concepts

Requires https://github.com/tree-sitter/tree-sitter-cpp/pull/138

tests/query/highlights/cpp/concepts.cpp

@@ -0,0 +1,22 @@
+
+template <class T, class U>
+concept Derived = std::is_base_of<U, T>::value;
+//  ^ keyword
+//       ^ type
+
+template<typename T>
+concept Hashable = requires(T a) {
+//                   ^ keyword
+//                            ^ parameter
+//                          ^ type
+    { std::hash<T>{}(a) } -> std::convertible_to<std::size_t>;
+ typename CommonType<T, U>; // CommonType<T, U> is valid and names a type
+    { CommonType<T, U>{std::forward<T>(t)} }; 
+    { CommonType<T, U>{std::forward<U>(u)} }; 
+};
+
+
+template<typename T>
+    requires requires (T x) { x + x; } // ad-hoc constraint, note keyword used twice
+//                 ^ keyword
+T add(T a, T b) { return a + b; }