[MISC] Add HIBF_UNREACHABLE

[eseiler]

Only used in one file in the library, but

• it may be used more in the future
• it's a nice utility for dependents

[seqan-actions]

Documentation preview available at https://docs.seqan.de/preview/seqan/hibf/269

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.57%. Comparing base (6e90be6) to head (5a04fa6).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #269      +/-   ##
==========================================
- Coverage   99.59%   99.57%   -0.03%     
==========================================
  Files          52       52              
  Lines        1996     1862     -134     
  Branches        5        5              
==========================================
- Hits         1988     1854     -134     
  Misses          8        8              

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