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 4.77%. Comparing base (999ae89) to head (d3ae0b6).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #259   +/-   ##
=====================================
  Coverage   4.77%   4.77%           
=====================================
  Files          8       8           
  Lines       1153    1153           
  Branches     255     255           
=====================================
  Hits          55      55           
  Misses      1097    1097           
  Partials       1       1           

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[benoit74]

LGTM