[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`.
Prepending with exec will break several cases, which are not prevented by your current patch:

- Windows (which does not have ``exec``)
- complex commands (using ``&&``, ``||`` or multiple commands separated by ``;``)
- enhanced sigchild compatibility mode (because of the previous case)