[Jekyll] Support other template engines via plugins

<!-- Hi! Thanks for considering to file a feature request with Jekyll. Please take the time to answer the basic questions. Please try to be as detailed as possible. Thanks! --> ## Summary <!-- A one-paragraph explanation of the feature. --> Add a new hook that works with layouts, so custom template engines can be implemented as pre/post processors to files. ## Motivation <!-- Why do you want to see this feature in Jekyll? What makes you sure that it should not be implemented at the plugin level but in Jekyll core? What use cases does it support? NOTE: Please be mindful of the Jekyll philosophy (https://jekyllrb.com/philosophy/), particularly Section 5. Think about if 90% of the users would benefit from your feature request and whether your feature would be better off in a plugin. --> GitHub has recently [made it quite easy to deploy to GitHub pages via actions](https://jekyllrb.com/docs/continuous-integration/github-actions/). With th