[feat] engine: re-enables z-library (zlibrary-global.se)

- re-enables z-library as the new domain zlibrary-global.se is now available
  from the open web.   The announcement of the domain:

    https://www.reddit.com/r/zlibrary/comments/13whe08/mod_note_zlibraryglobalse_domain_is_officially/

  It is an official domain, it requires to log in to the "personal" subdomain
  only to download files, but the search works.

- changes the result template of zlibrary to paper.html, filling the appropriate fields
- implements language filtering for zlibrary
- implement zlibrary custom filters (engine traits)
- refactor and document the zlibrary engine