buffer: add fast api for isAscii & isUtf8

[anonrig]

Adds v8 fast api for IsUtf8 and IsAscii methods

isAscii benchmark: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1706/

                                                                        confidence improvement accuracy (*)   (**)  (***)
buffers/buffer-isascii.js input='hello world' length='long' n=20000000         ***     37.08 %       ±0.87% ±1.17% ±1.52%
buffers/buffer-isascii.js input='hello world' length='short' n=20000000        ***     36.83 %       ±0.86% ±1.15% ±1.51%

isUtf8 benchmark: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1708/

buffers/buffer-isutf8.js input='∀x∈ℝ: ⌈x⌉ = −⌊−x⌋' length='long' n=20000000         ***      1.95 %       ±0.22% ±0.29% ±0.38%
buffers/buffer-isutf8.js input='∀x∈ℝ: ⌈x⌉ = −⌊−x⌋' length='short' n=20000000        ***     65.95 %       ±2.35% ±3.13% ±4.08%
buffers/buffer-isutf8.js input='regular string' length='long' n=20000000            ***     42.16 %       ±4.05% ±5.39% ±7.02%
buffers/buffer-isutf8.js input='regular string' length='short' n=20000000           ***    121.41 %       ±1.15% ±1.54% ±2.02%

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.21%. Comparing base (e0cf8ae) to head (ab8d328).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	90.47%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #58058      +/-   ##
==========================================
+ Coverage   90.18%   90.21%   +0.03%     
==========================================
  Files         630      630              
  Lines      186393   186410      +17     
  Branches    36595    36612      +17     
==========================================
+ Hits       168103   168175      +72     
+ Misses      11090    11058      -32     
+ Partials     7200     7177      -23     

Files with missing lines	Coverage Δ
src/node_buffer.cc	67.92% <90.47%> (+0.41%)	⬆️

... and 32 files with indirect coverage changes

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[mcollina]

lgtm

[lemire]

Great work.

[Renegade334]

This doesn't account for any offset on the backing ArrayBuffer. ArrayBufferViewContents is probably needed here.