rename length constraints to graphemes

[elfkuzco]

Changes

• rename length constraints to end with graphemes (See refactor grapheme string validation for offliner flags zimfarm#1890)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.90%. Comparing base (e939644) to head (79084ad).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #467   +/-   ##
=====================================
  Coverage   0.90%   0.90%           
=====================================
  Files         11      11           
  Lines       1211    1211           
  Branches     166     166           
=====================================
  Hits          11      11           
  Misses      1200    1200           

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[benoit74]

LGTM