src: constrain MaybeStackBuffer::ToString and ToStringView to standard char types

[om-ghante]

On newer libc++ (shipped with macOS Xcode 16+), std::char_traits<T> for T not
equal to char, wchar_t, char8_t, char16_t, or char32_t is deprecated and
will be removed in a future release.

When the MaybeStackBuffer template is instantiated with unsigned char or uint8_t
(e.g., in test/cctest/test_util.cc), the compiler instantiates the declarations
for all member functions. The ToString() and ToStringView() methods historically returned
std::basic_string<T> and std::basic_string_view<T>, which triggered this
deprecation warning (-Wdeprecated-declarations) during class instantiation because
the return type references std::char_traits<unsigned char>.

Changes

This PR resolves the warning by:

1. Extracting the list of standard character types into a reusable C++20 concept
   standard_char_type.
2. Converting ToString() and ToStringView() from normal member functions into
   member function templates with a constrained default template parameter:
   template <standard_char_type U = T>.

By making these member function templates, the C++ compiler defers instantiating
the return type (std::basic_string<U>) until the function is actually called. Since
these methods are never actually called for unsigned char or uint8_t variants
in the Node.js codebase, the deprecated std::char_traits<unsigned char> is never
evaluated or instantiated, completely bypassing the warning.

Refs: #62506

[targos]

Have you tried your changes? I applied the patch and still see a deprecation warning.

[om-ghante]

Have you tried your changes? I applied the patch and still see a deprecation warning.

Yes, I have checked my updated changes and the deprecation warning is now gone.

The issue with the first patch was that requires only stops the function from being callable, but the return type std::basic_string is still evaluated during class instantiation, which triggers the warning. By changing ToString() and ToStringView() to member function templates (template <standard_char_type U = T>), the compiler defers instantiating the return type (std::basic_string<U>) until the method is actually called. Since these methods are never used for MaybeStackBuffer<unsigned char>, the deprecation warning is completely bypassed.

[targos]

Thanks. I confirm there is no warning anymore with the latest push. I defer to @nodejs/cpp-reviewers for the code review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (d9645d7) to head (9f3892d).
⚠️ Report is 37 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62507      +/-   ##
==========================================
- Coverage   89.69%   89.69%   -0.01%     
==========================================
  Files         692      695       +3     
  Lines      214039   214214     +175     
  Branches    41064    41026      -38     
==========================================
+ Hits       191985   192140     +155     
- Misses      14134    14155      +21     
+ Partials     7920     7919       -1     

Files with missing lines	Coverage Δ
src/util.h	90.98% <100.00%> (+0.07%)	⬆️

... and 80 files with indirect coverage changes

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