Make packages.json synchronization optional

[FrancoisPog]

There are cases where a project uses Yarn workspaces and Javascript dependencies may be in multiple packages.json files, not just the one at the root.
In this case we don't want them added.

[dmaicher]

I have an issue where symfony/stimulus-bundle is a transitive dependency and I don't use it for my own assets. So I don't want my package.json to be modified in any way. So 👍 for this option.

For now I prevent any update to package.json with this ugly workaround in composer.json:

    "scripts": {
        "pre-install-cmd": "test -e 'package.json' && mv package.json _package.json || exit 0",
        "pre-update-cmd": "test -e 'package.json' && mv package.json _package.json || exit 0",
        "post-install-cmd": "mv _package.json package.json",
        "post-update-cmd": "mv _package.json package.json"
    },