Integration with Symfony Mailer

Here is just re-rolled patch of #5

Since the Symfony Mailer API has changed a bit (https://git.drupalcode.org/project/symfony_mailer/-/commit/f66c09572acd7...) I adjusted #6

We are sending HTML emails so here is a quick fix to allow commerce_reactions return render array to consume later by mailer.
Not a best solution but works for the moment.

Here is a patch with using renderPlain() instead of render() because sometimes we're having the next error:

LogicException: Render context is empty, because render() was called outside of a renderRoot() or renderPlain() call