Support []Flaggable flags where multiple uses append to the underlying slice

[FollowTheProcess]

• Viable pattern for slice flags
• Add []int8

Summary

[codecov]

Codecov Report

Attention: Patch coverage is 82.32759% with 41 lines in your changes missing coverage. Please review.

Project coverage is 90.75%. Comparing base (5e0d86f) to head (a9d81f2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/flag/flag.go	82.32%	29 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
- Coverage   91.88%   90.75%   -1.13%     
==========================================
  Files           6        6              
  Lines        1367     1590     +223     
==========================================
+ Hits         1256     1443     +187     
- Misses         79      103      +24     
- Partials       32       44      +12     

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