[Process] add a method to the ProcessBuilder to automatically prepend the executed script with "exec"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3999

This pull requests addresses the issue raised by @romainneutron in #5759. It eases the process of creating a ``Process`` whose script should be prepended by `exec`.
From what I see, the ``ProcessBuilder`` cannot create multiple commands. Running something like this:

```php
$builder = new ProcessBuilder(array(
    'php',
    './test1.php',
    ';',
    'php',
    './test2.php',
));
```

passes ``;``, ``php`` and ``./test2.php`` as arguments to the ``test1.php`` script.