We already allow a comma at the end of initialiser lists and enums,
and so I propose that we also allow a trailing comma in the following:

    int a, b, c, ;

The reason why I want that comma after 'c' is that sometimes I write
a source file as follows:

std::jthread
    thread_listener,
     thread_sender,
     thread_monitor
     ;

When I add a new variable called 'thread_socket', I want the diff file
in SVN and Git to simply show that I've added a new line. But instead,
it shows that I've edited one line and added one line, as follows:

- thread_monitor
+ thread_monitor,
+ thread_socket