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 ./buffer-result.csv
confidence improvement accuracy (*) (**) (***)
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=1024 0.34 % ±1.92% ±2.56% ±3.35%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=16384 -0.57 % ±1.01% ±1.34% ±1.74%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=262144 0.80 % ±1.13% ±1.51% ±1.96%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=4194304 * 0.52 % ±0.46% ±0.61% ±0.80%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='ascii' size=64 9.85 % ±30.50% ±40.59% ±52.85%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1_then_cjk' size=1024 *** 5.25 % ±1.60% ±2.13% ±2.78%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1_then_cjk' size=16384 *** 24.32 % ±0.71% ±0.95% ±1.24%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1_then_cjk' size=262144 *** 24.33 % ±0.86% ±1.15% ±1.51%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1_then_cjk' size=4194304 -0.06 % ±0.53% ±0.71% ±0.92%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1_then_cjk' size=64 -2.30 % ±2.95% ±3.93% ±5.13%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=1024 *** 9.02 % ±3.85% ±5.17% ±6.82%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=16384 *** 34.52 % ±1.42% ±1.90% ±2.50%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=262144 *** 30.06 % ±1.48% ±1.98% ±2.62%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=4194304 0.09 % ±0.42% ±0.56% ±0.73%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='latin1' size=64 * 6.60 % ±5.00% ±6.70% ±8.82%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=1024 *** 6.47 % ±1.88% ±2.50% ±3.27%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=16384 *** 33.69 % ±1.62% ±2.16% ±2.82%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=262144 *** 47.29 % ±1.09% ±1.45% ±1.89%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=4194304 -0.31 % ±0.61% ±0.82% ±1.07%
buffers/buffer-tostring-utf8-latin1.js n=10000 content='utf8_mixed' size=64 * -2.47 % ±2.25% ±2.99% ±3.89%
Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 20 comparisons, you can thus expect the following amount of false-positive results:
1.00 false positives, when considering a 5% risk acceptance (*, **, ***),
0.20 false positives, when considering a 1% risk acceptance (**, ***),
0.02 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 68.25397% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.06%. Comparing base (265679b) to head (55f6ae9).
⚠️ Report is 189 commits behind head on main.

Files with missing lines	Patch %	Lines
src/string_bytes.cc	68.25%	14 Missing and 6 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #63385 +/- ##
==========================================
+ Coverage 90.05% 90.06% +0.01% 
==========================================
Files 714 714 Lines 225628 225973 +345 Branches 42673 42761 +88 ==========================================
+ Hits 203198 203533 +335 + Misses 14225 14212 -13 - Partials 8205 8228 +23 

Files with missing lines	Coverage Δ
src/string_bytes.cc	73.21% <68.25%> (-1.42%)	⬇️

... and 57 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mertcanaltin]

Also covers @addaleax's review on #63370, fs picks it up through the shared StringBytes path

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73826/

[gurgunday]

lgtm

[nodejs-github-bot]

Landed in 6813080