executors: add C++23 support

dmoj/executors/CPP23.py

@@ -0,0 +1,18 @@
+from dmoj.executors.c_like_executor import CPPExecutor, GCCMixin
+
+
+class Executor(GCCMixin, CPPExecutor):
+    command = 'g++23'
+    command_paths = ['g++-13', 'g++']
+    std = 'c++23'
+    test_program = """
+#include <iostream>
+
+#if __cplusplus >= 202302L
+int main() {
+    auto input = std::cin.rdbuf();
+    std::cout << input;
+    return 0;
+}
+#endif
+"""