PHAR runtime #135

Requires a few tweaks: symfony/symfony#49443

Some of these could be solved in the runtime instead of relying on the user to do them: https://github.com/box-project/box/blob/main/doc/symfony.md#symfony-support

Runtime would need to ship:

1. itself
2. the decorated Symfony Kernel which accepts passing the project root to it, which it then returns via getProjectRoot()
3. the fixed runtime template to allow supporting older versions of Symfony since they're likely to mislabel the bugfix for the existing template as a feature

/cc @theofidry