Add --related-items, JSON output, and better testing to lenskit recommend

[mdekstrand]

This primarily adds --related-items and --json to the LensKit recommend CLI command.

Support code for using #1086.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.68%. Comparing base (dc34cae) to head (78ab1b3).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/cli/recommend.py	57.14%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1087      +/-   ##
==========================================
+ Coverage   89.49%   89.68%   +0.19%     
==========================================
  Files         231      231              
  Lines       16017    16042      +25     
==========================================
+ Hits        14334    14388      +54     
+ Misses       1683     1654      -29     

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