refactor: split add/del/set code into files

[tri-adam]

Split add/del/set code (and associated tests) into separate files. No code changes, purely moving code in this one.

[codecov]

Codecov Report

Attention: Patch coverage is 64.70588% with 78 lines in your changes missing coverage. Please review.

Project coverage is 62.65%. Comparing base (f2f07f5) to head (84ca397).

Files	Patch %	Lines
pkg/sif/set.go	50.00%	32 Missing and 16 partials ⚠️
pkg/sif/delete.go	76.13%	14 Missing and 7 partials ⚠️
pkg/sif/add.go	75.67%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #364   +/-   ##
=======================================
  Coverage   62.65%   62.65%           
=======================================
  Files          34       37    +3     
  Lines        3245     3245           
=======================================
  Hits         2033     2033           
  Misses       1046     1046           
  Partials      166      166           

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[dtrudg]

LGTM