Fix the parsing of long flag strings

[tgross35]

Long flag strings such as "ABCD" were getting parsed as "AA", "BB", "CC", "DD", rather than the correct "ABCD". Fix this, add a unit and integration test, and do some mild reorganization.

Should improve part of #108

[codecov]

Codecov Report

Attention: Patch coverage is 88.39286% with 13 lines in your changes missing coverage. Please review.

Project coverage is 76.77%. Comparing base (fe17d09) to head (fdd2cc9).

Files	Patch %	Lines
zspell/src/dict/flags.rs	55.55%	12 Missing ⚠️
zspell/test-util/src/lib.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   76.17%   76.77%   +0.59%     
==========================================
  Files          27       27              
  Lines        3308     3371      +63     
==========================================
+ Hits         2520     2588      +68     
+ Misses        788      783       -5     

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