Trailing commas in function calls

Arrays have allowed a trailing comma since forever and calls did not, which is why a multi-line call always produced a two-line diff when an argument was added.

$container->register(
    OrderRepository::class,
    $connection,
    $logger,
);

// still a parse error in 7.2: the declaration
function register($class, $connection, $logger,) {}   // 8.0

The benefit is entirely in review — one added line rather than one added and one modified, so the diff shows what changed. Declarations did not get it in this release, which is the asymmetry to remember when a linter complains about a signature. It is worth turning on in the coding standard immediately, because retrofitting it later touches every multi-line call in the codebase.