chore(cpp): add frequently used headers and yes/no

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

.common/cpp/skeleton.cpp

@@ -1,4 +1,13 @@
+#include <algorithm>
+#include <cassert>
+#include <cctype>
+#include <cstdint>
+#include <functional>
 #include <iostream>
+#include <optional>
+#include <sstream>
+#include <string>
+#include <vector>
 
 namespace helpers {
 
@@ -16,6 +25,9 @@ inline void answer(const T &ans) {
   cout << ans << "\n";
 }
 
+inline void yes() { cout << "YES\n"; }
+inline void no() { cout << "NO\n"; }
+
 }  // namespace helpers
 
 namespace {