Add patchLevel for drupal/core to extra section of composer.json

[balsama]

composer-patches ends up leaving a stray directory after trying to apply patches to drupal/core in the wrong place. Lightning Project should use the patchLevel key to declare that drupal/core patches should be applied as -p2.

I.e.:

        "patchLevel": {
            "drupal/core": "-p2"
        }