Preserve multibyte characters when wrapping long source lines

[fhammerschmidt]

Fixes https://forum.rescript-lang.org/t/rewatch-12-3-0-panics-with-a-multichar-em-dash-on-a-line-that-bsc-warnings-wraps/7426/1

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.70%. Comparing base (fc2c00f) to head (37fece4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8520   +/-   ##
=======================================
  Coverage   74.70%   74.70%           
=======================================
  Files         458      458           
  Lines       62360    62366    +6     
=======================================
+ Hits        46587    46593    +6     
  Misses      15773    15773           

Files with missing lines	Coverage Δ
compiler/ml/code_frame.ml	94.53% <100.00%> (+0.17%)	⬆️
tests/ounit_tests/ounit_utf8_test.ml	100.00% <100.00%> (ø)

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