Replace eslint-plugin-json with a fixture processor

[rasmi]

While updating to support eslint v10 in #3230, eslint-plugin-json (and specifically eslint-plugin-jsonc for v10+) caused a series of interrelated version/dependency issues which required various dep-time-travel.sh and version-pinning workarounds (see here).

The #1645 regression test only needs some .json processor in the pipeline so no-unused-modules is exercised against a JSON-only directory (the bug was the import rule crashing there, not the JSON diagnostic). The fixture does that and removes the JSON-plugin dependency entirely.

(potential alternative to #3230's eslint-plugin-jsonc switch d8d7294)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.77%. Comparing base (1ab0c39) to head (7b153c2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3252      +/-   ##
==========================================
+ Coverage   95.50%   95.77%   +0.27%     
==========================================
  Files          83       83              
  Lines        3692     3693       +1     
  Branches     1335     1336       +1     
==========================================
+ Hits         3526     3537      +11     
+ Misses        166      156      -10     

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[rasmi]

Will remove the workarounds in #3230 today and rebase on main!