fs: add c++ fast path for writeFileSync string utf8

[CanadaHonk]

Summary

Added fast path in almost entirely C++ for writeFileSync with UTF8 encoding and string data. Also improves appendFileSync as it just uses writeFileSync under the hood. Only string data as Buffer seems questionable in benchmarks for this so I'll just leave it for strings for now.

TL;DR: This makes writeFileSync(path, data: string) UTF8 up to ~2.5x faster depending on data size, especially when using file descriptors

Bench results

Benchmark in this PR

Note: running locally on Linux/i9/SSD

                                                                                                                 confidence improvement accuracy (*)   (**)  (***)
fs/bench-writeFileSync.js n=1000 func='writeFile' useBuffer='false' length=1024 useFd='false' encoding='utf8'                   -0.37 %       ±4.40% ±5.90% ±7.77%
fs/bench-writeFileSync.js n=1000 func='writeFile' useBuffer='false' length=1024 useFd='true' encoding='utf8'            ***     49.15 %       ±5.11% ±6.82% ±8.91%
fs/bench-writeFileSync.js n=1000 func='writeFile' useBuffer='false' length=102400 useFd='false' encoding='utf8'         ***      6.69 %       ±1.67% ±2.22% ±2.89%
fs/bench-writeFileSync.js n=1000 func='writeFile' useBuffer='false' length=102400 useFd='true' encoding='utf8'          ***     74.52 %       ±4.35% ±5.85% ±7.74%
fs/bench-writeFileSync.js n=1000 func='writeFile' useBuffer='false' length=1048576 useFd='false' encoding='utf8'        ***     16.05 %       ±1.60% ±2.14% ±2.81%
fs/bench-writeFileSync.js n=1000 func='writeFile' useBuffer='false' length=1048576 useFd='true' encoding='utf8'         ***     85.82 %       ±3.58% ±4.77% ±6.23%

Benchmark CI (old): https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1421/

Alternative benchmark

Alternative benchmark using Bun's bench for fs.copyFileSync (string data, using paths)

Current (main)

benchmark           time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------ -----------------------------
12 ascii          2.19 µs/iter     (2.07 µs … 2.67 µs)   2.21 µs   2.67 µs   2.67 µs
12 utf8           2.23 µs/iter     (2.08 µs … 2.32 µs)   2.26 µs   2.32 µs   2.32 µs
12288 ascii       6.14 µs/iter     (5.97 µs … 6.62 µs)   6.14 µs   6.62 µs   6.62 µs
18432 utf8       40.02 µs/iter  (36.13 µs … 204.16 µs)  40.05 µs   63.6 µs  69.01 µs

This PR

Long ascii: 6.14µs -> 3.46µs (~1.8x speedup)
Long utf8: 40.02µs -> 18.25µs (~2.2x speedup)

benchmark           time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------ -----------------------------
12 ascii          1.97 µs/iter      (1.9 µs … 2.03 µs)   1.98 µs   2.03 µs   2.03 µs
12 utf8           1.95 µs/iter        (1.91 µs … 2 µs)   1.97 µs      2 µs      2 µs
12288 ascii       3.46 µs/iter      (3.39 µs … 3.6 µs)   3.47 µs    3.6 µs    3.6 µs
18432 utf8       18.25 µs/iter  (16.06 µs … 147.95 µs)  18.48 µs  21.77 µs  24.03 µs