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 33.33%. Comparing base (d6bd5bc) to head (780625a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #149   +/-   ##
=======================================
  Coverage   33.33%   33.33%           
=======================================
  Files          11       11           
  Lines         477      477           
  Branches       92       92           
=======================================
  Hits          159      159           
  Misses        313      313           
  Partials        5        5           

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[benoit74]

LGTM