[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

searx/sxng_locales.py

@@ -41,6 +41,7 @@ sxng_locales = (
     ('es-US', 'Español', 'Estados Unidos', 'Spanish', '\U0001f1fa\U0001f1f8'),
     ('et', 'Eesti', '', 'Estonian', '\U0001f310'),
     ('et-EE', 'Eesti', 'Eesti', 'Estonian', '\U0001f1ea\U0001f1ea'),
+    ('fa', 'فارسی', '', 'Persian', '\U0001f310'),
     ('fi', 'Suomi', '', 'Finnish', '\U0001f310'),
     ('fi-FI', 'Suomi', 'Suomi', 'Finnish', '\U0001f1eb\U0001f1ee'),
     ('fr', 'Français', '', 'French', '\U0001f310'),