src: add Latin1 fast path in StringBytes::Encode utf8

[mertcanaltin]

In StringBytes::Encode utf8, latin1-fits content was going through the UTF-16 path. I added a latin1 fast path that converts via simdutf and returns a one-byte V8 string. Benefits every Buffer.toString('utf8'), including fs.readFile/promises.readFile when they delegate to it.

@nodejs/performance @mcollina @anonrig @lemire @addaleax

Benchmark results:

➜  node git:(mert/buffer-tostring-utf8-latin1) ✗ node-benchmark-compare ./result.csv
                                                                                 confidence improvement accuracy (*)   (**)  (***)
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=1024                         0.96 %       ±1.54% ±2.06% ±2.68%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=16384                        0.68 %       ±1.03% ±1.37% ±1.78%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=262144                      -0.87 %       ±1.82% ±2.42% ±3.16%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=4194304                      0.06 %       ±0.65% ±0.87% ±1.14%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=64                          -1.13 %       ±1.66% ±2.21% ±2.89%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=1024               ***     12.38 %       ±1.04% ±1.39% ±1.81%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=16384              ***     39.65 %       ±1.96% ±2.63% ±3.47%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=262144             ***     40.57 %       ±1.10% ±1.48% ±1.96%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=4194304            ***      2.78 %       ±0.60% ±0.80% ±1.05%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=64                          0.22 %       ±1.23% ±1.65% ±2.16%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=1024                    1.33 %       ±1.41% ±1.87% ±2.44%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=16384                  -0.30 %       ±1.00% ±1.33% ±1.73%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=262144                 -0.51 %       ±1.27% ±1.70% ±2.23%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=4194304                -0.26 %       ±0.67% ±0.89% ±1.16%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=64                      1.07 %       ±1.66% ±2.21% ±2.88%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 15 comparisons, you can thus expect the following amount of false-positive results:
  0.75 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.15 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)
➜  node git:(mert/buffer-tostring-utf8-latin1) ✗

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.07%. Comparing base (265679b) to head (f2a6a98).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/string_bytes.cc	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63385      +/-   ##
==========================================
+ Coverage   90.05%   90.07%   +0.01%     
==========================================
  Files         714      714              
  Lines      225628   225647      +19     
  Branches    42673    42691      +18     
==========================================
+ Hits       203198   203248      +50     
+ Misses      14225    14186      -39     
- Partials     8205     8213       +8     

Files with missing lines	Coverage Δ
src/string_bytes.cc	75.63% <94.44%> (+0.99%)	⬆️

... and 36 files with indirect coverage changes

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