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fs: use fast api calls for `existsSync`

Rodrigo Muino Tomonari requested to merge github/fork/littledivy/perf-existsSync into main

Currently, It takes the fast route when path string is represented as a OneByteString in V8.

                                                          confidence improvement accuracy (*)   (**)  (***)
fs/bench-existsSync.js n=1000000 type='existing'                 ***      2.87 %       ±0.68% ±0.91% ±1.19%
fs/bench-existsSync.js n=1000000 type='non-existing'             ***     43.04 %       ±1.17% ±1.56% ±2.03%
fs/bench-existsSync.js n=1000000 type='non-flat-existing'        ***      2.63 %       ±0.42% ±0.56% ±0.73%
n=1000000 type='non-existing' 836103.841448331                    NA       NaN %           NA     NA     NA

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 4 comparisons, you can thus expect the following amount of false-positive results:
  0.20 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.04 false positives, when considering a   1% risk acceptance (**, ***),
  0.00 false positives, when considering a 0.1% risk acceptance (***)

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