Attach libraries directly from a template file

As mentioned, the libraries inside the *.info.yml file are include globally, on every page. To conditionally add a library, a preprocess function (#attached), just like in Drupal 7, had to be used. In order to improve the front-end developer experience, a simple Twig function has been added to easify this. {{ attach_library('theme/library') }} makes it possible to add libraries directy from template files. A huge win.

The only template files that doesn't support this is html.html.twig. More information can be found here.