title: Add new environment variables to .env when installing a package
body: <p>In our company, several internal projects rely on the same copied code parts to handle connections to the same APIs (like: Google Suite, JIRA,...). To avoid copying the same code over and over again for new projects, I want to create Symfony packages that collect these API classes.</p>
  <p>The tricky part: I'm looking for a way to add the neccessary env variables automatically to <code>.env</code>, just like <a href="https://github.com/symfony/recipes#env-configurator" rel="nofollow noreferrer">Symfony's recipe structure does it</a>. But as these projects should only be used internally, pushing their recipe configuration to a public repository is a no-go for me. Adding a custom recipe server (like the one by <a href="https://github.com/moay/server-for-symfony-flex" rel="nofollow noreferrer">moay</a>) looks interesting to me, but needs additional configuration in each projects <code>composer.json</code>.</p>
  <p>Is there any better way to resolve this, such that I could simply define the needed variables solely in my project, such that they get added to <code>.env</code> without any additional magic?</p>
  <p>NB: anything that requires <code>symfony/flex</code> is fine, as this should be part of all new projects in our company</p>
  <p>These are solutions I want to avoid:</p>
  <ul>
  <li>add configuration to bundles / packages itself, such that these configuration values are put under version control</li>
  <li>add configuration through any other command that is run <strong>manually</strong> after installing</li>
  </ul>