Add stop() method to Process and Pool

[MiniCodeMonkey]

I had a process pool:

$this->pool = Process::pool(function (Pool $pool) {
    $pool->path(base_path())->command('sleep 5');
    $pool->path(base_path())->command('sleep 10');
})->start();

And I needed to stop all processes in the pool gracefully if they were running.

I was reaching for:

$this->pool->stop()

...which didn't exist, so I decided to add it.

We already have $this->pool->signal(...), but the Symfony stop() method adds some additional logic to ensure that the process is gracefully shut down (if possible): https://github.com/symfony/process/blob/7.1/Process.php#L908-L935

There doesn't seem to be any relevant test cases to add here, the main thing is the actual logic if stop() which is covered by https://github.com/symfony/process/blob/7.1/Tests/ProcessTest.php#L130