Add a third fuzzer for @/$ equivalency

[MichaelChirico]

Part of #2191 -- continuing #2819 (which is included here & against which this is based, for now). Also progress on #2737.

This one caught a pretty big gap in our current linter logic, as noted in #2820. Those are addressed here.

I left the new include_s4_slots=FALSE by default since apparently TRUE is only needed in one place to pass the current fuzz suite.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (fe88c59) to head (c3a99ce).

Additional details and impacted files

@@            Coverage Diff             @@
##           fuzz-pipe    #2821   +/-   ##
==========================================
  Coverage      99.64%   99.64%           
==========================================
  Files            126      126           
  Lines           6961     6978   +17     
==========================================
+ Hits            6936     6953   +17     
  Misses            25       25           

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