Create 2.x branch and release 2.x-dev

Thank you for this great module!!

Now that there's a 2.1.0 release it's obvious that a 2.x branch is needed. Please create it.

And for easier implementation of the dev version it's also necessary to release this branch so it becomes available for easy installation via Composer. I want to be able to just run composer require 'drupal/media_entity_lottie:2.x-dev@dev' to install the develop version in a project.