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 35.33%. Comparing base (d213e5d) to head (862d779).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #112   +/-   ##
=======================================
  Coverage   35.33%   35.33%           
=======================================
  Files          11       11           
  Lines        1033     1033           
  Branches      147      147           
=======================================
  Hits          365      365           
  Misses        652      652           
  Partials       16       16           

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[benoit74]

LGTM