fix(list): highlight symbol when using a multi-bytes char

[mrjackwills]

ratatui v0.26.0 brought a regression in the List widget, in which the highlight symbol width was incorrectly calculated - specifically when the highlight symbol was a multi-char character, e.g. ▶.

I have also added a single additional test to check the output when such a multi-byte char is used for the highlight symbol.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (61a8278) 92.0% compared to head (863ffc3) 92.0%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #924   +/-   ##
=====================================
  Coverage   92.0%   92.0%           
=====================================
  Files         61      61           
  Lines      15493   15493           
=====================================
  Hits       14259   14259           
  Misses      1234    1234           

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[kdheepak]

Nice find!