feat: add ability to skip package.json synchronization

[wuchen90]

For those who doesn't want the package.json synchronization, this PR offers to opt-out this behavior by setting the config below in extra section of composer.json:

{
  "extra": {
    "symfony/flex": {
      "synchronize_package_json": false
    }
  }
}

fix #1017

[skmedix]

Perhaps it would be better if it logs what exactly has been skipped?

[wuchen90]

Perhaps it would be better if it logs what exactly has been skipped?

@skmedix Currently it already outputs this <info>Skip synchronizing package.json with PHP packages</>. What kind of logs do you have in mind?

[dmaicher]

I also need this option 👍

[skmedix]

I think that's good enough, so LGTM

[fabpot]

Thank you @wuchen90.