chore(cpp): add RNG generator to skeleton

For details see: https://codeforces.com/blog/entry/61675

Signed-off-by: Matej Focko <me@mfocko.xyz>

.common/cpp/skeleton.cpp

@@ -258,6 +258,10 @@ inline void no() { std::cout << "NO\n"; }
 inline void yesno(bool ans) { std::cout << (ans ? "YES" : "NO") << "\n"; }
 #pragma endregion /* output */
 
+#pragma region rng
+std::mt19937 rng(std::chrono::steady_clock::now().time_since_epoch().count());
+#pragma endregion /* rng */
+
 #pragma endregion /* helpers */
 
 // for ‹N› test cases, uncomment for single test case