fix: codon deletion right in front of stop codon is not a stop_gained

[tedil]

This also changes the way conservative is determined; I am not sure whether the end position is in- or exclusive at that point, i.e. if the change here is correct or not.
Edit: should indeed be end_base + 1

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.97%. Comparing base (2431c21) to head (ffdd805).
Report is 5 commits behind head on main.

Files	Patch %	Lines
src/annotate/seqvars/csq.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
+ Coverage   73.94%   73.97%   +0.03%     
==========================================
  Files          25       25              
  Lines        9656     9665       +9     
==========================================
+ Hits         7140     7150      +10     
+ Misses       2516     2515       -1     

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