buffer: add buffer.stringLength()

[mcollina]

Add buffer.stringLength(input[, encoding]), the counterpart of Buffer.byteLength(): it returns the number of UTF-16 code units that buf.toString(encoding) would produce, without decoding.

For UTF-8 the count is computed with simdutf. Invalid input is counted with the same maximal-subpart replacement the decoder applies, so the result always matches toString().length. The other encodings are computed from byteLength alone.

This lets code that accumulates streamed input check the result against buffer.constants.MAX_STRING_LENGTH and size its memory budget before decoding, as discussed in #66062.

Throughput on x64 (benchmark/buffers/buffer-stringlength.js, utf8):

input	4 KiB	1 MiB
ascii	3.5M ops/s	~17 GB/s
multibyte valid	1.7M ops/s	~7.5 GB/s
invalid at the end	1.6M ops/s	~7.5 GB/s

Refs: #66062

--

AI generated. Humanly reviewed.

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[codecov]

Codecov Report

❌ Patch coverage is 90.42553% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (7aaf9b4) to head (b1c6e2b).
⚠️ Report is 438 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	86.15%	5 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66064      +/-   ##
==========================================
+ Coverage   90.13%   90.27%   +0.13%     
==========================================
  Files         751      789      +38     
  Lines      253639   271568   +17929     
  Branches    47790    51833    +4043     
==========================================
+ Hits       228618   245156   +16538     
- Misses      16264    16880     +616     
- Partials     8757     9532     +775     

Files with missing lines	Coverage Δ
lib/buffer.js	99.74% <100.00%> (+0.59%)	⬆️
src/node_buffer.cc	70.23% <86.15%> (+0.30%)	⬆️

... and 290 files with indirect coverage changes

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