Add JSON export to item lists and item list collections

[mdekstrand]

This adds methods to export ItemList and ItemListCollection to JSON-compatible data (or JSON strings).

[codecov]

Codecov Report

❌ Patch coverage is 92.68293% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.49%. Comparing base (adb9361) to head (dc34cae).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/data/_adapt.py	85.71%	1 Missing ⚠️
src/lenskit/data/_collection/_base.py	93.33%	1 Missing ⚠️
src/lenskit/data/_items.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1089      +/-   ##
==========================================
+ Coverage   89.45%   89.49%   +0.03%     
==========================================
  Files         231      231              
  Lines       15983    16017      +34     
==========================================
+ Hits        14298    14334      +36     
+ Misses       1685     1683       -2     

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