Add a fourth fuzzer for injecting comments

[MichaelChirico]

Part of #2191 -- continuing #2821 (which is included here & against which this is based, for now). Also progress on #2737.

Closes #2824. Closes #2825. Closes #2826. Closes #2827.

Wow, this one really spiralled out of control & became quite a quixotic endeavor in the end.

A lot of it is really edge-casey, but better to have the fix than not. A surprising number of legit issues / bugs (i.e. not false positive/negative, but {lintr} errors based on faulty assumptions).

Probably, some pieces of this belong in dedicated PRs separate from this one.

Filing for now to get things started.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (c3a99ce) to head (a42bda5).

Additional details and impacted files

@@             Coverage Diff              @@
##           fuzz-dollar    #2822   +/-   ##
============================================
  Coverage        99.64%   99.64%           
============================================
  Files              126      126           
  Lines             6978     7041   +63     
============================================
+ Hits              6953     7016   +63     
  Misses              25       25           

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