src: refactor WriteUCS2 and remove flags argument

[tniessen]

This change refactors StringBytes::WriteUCS2() in multiple ways.

The flags argument being passed to WriteUCS2() is not useful: the only really relevant flag is NO_NULL_TERMINATION since V8 ignores REPLACE_INVALID_UTF8, HINT_MANY_WRITES_EXPECTED, and PRESERVE_ONE_BYTE_NULL for UTF-16 strings. However, WriteUCS2() might not null-terminate the result correctly regardless of whether NO_NULL_TERMINATION is set because it makes multiple calls to String::Write() internally. For these reasons, this patch removes the flags argument entirely and always assumes NO_NULL_TERMINATION.

Next, this patch replaces the calls to the deprecated function String::Write() with calls to the new function String::WriteV2(), which always succeeds and always writes a predictable number of characters, removing the need to deal with a return value here.

Lastly, this patch simplifies the implementation of WriteUCS2() and computes the exact number of characters nchars from the beginning, removing the need to later check again if the number of characters is zero.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.17%. Comparing base (92102c0) to head (ea0a7c6).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/string_bytes.cc	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58163      +/-   ##
==========================================
- Coverage   90.18%   90.17%   -0.02%     
==========================================
  Files         630      630              
  Lines      186473   186499      +26     
  Branches    36612    36614       +2     
==========================================
  Hits       168169   168169              
- Misses      11116    11120       +4     
- Partials     7188     7210      +22     

Files with missing lines	Coverage Δ
src/string_bytes.h	80.00% <ø> (ø)
src/string_bytes.cc	70.41% <92.30%> (+0.89%)	⬆️

... and 32 files with indirect coverage changes

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