Add support for ZIM aliases alongside redirects

[pawank925]

Add support for ZIM aliases, mirroring the existing redirect support.

• Creator.add_alias() in zim/creator: adds an alias (shares the target item content, unlike a redirect) with optional title, is_front and duplicate_ok handling, matching the add_redirect API.
• add_aliases_to_zim() in zim/filesystem: adds aliases from a list of (source, target, title) tuples and/or a zimwriterfs-compatible aliases TSV file.
• make_zim_file() exposes new aliases and aliases_file parameters.
• Deduplicator now uses the new Creator.add_alias() instead of the libzim-inherited method, so deduplicated entries get a proper title and front-article hint.

Fix #148

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (936498a) to head (4ab0081).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #355   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           42        42           
  Lines         2729      2761   +32     
  Branches       406       415    +9     
=========================================
+ Hits          2729      2761   +32     

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[pawank925]

@benoit74