buffer: search utf16le at every byte offset

[inoway46]

Fixes: #26448
Refs: #26594

Background

UTF-16LE/UCS2 searches currently treat the Buffer as an array of 16-bit
values. This rounds odd byte offsets down and only searches positions aligned
to the start of the Buffer.

For example:

const buf = Buffer.from('\u6881\u6882\u6881', 'utf16le');

buf.indexOf('\u6881', 1, 'utf16le');
// Current: 0
// Expected: 4

This change encodes string needles using the requested encoding and compares
the resulting bytes at every byte offset. Buffer and Uint8Array needles are
compared byte-for-byte in their entirety. It applies to the shared
implementation used by indexOf(), lastIndexOf(), and includes().

The Buffer#indexOf() documentation
defines encoding as determining the binary representation of string needles,
while Buffer and Uint8Array needles are used in their entirety. This change
also makes their implementation consistent with that contract.

Compatibility

This is a semver-major behavior change.

A UTF-16LE byte sequence can now match at an unaligned byte offset, crossing
code unit boundaries even when the search starts at an even offset. Such a
match may be returned before an aligned match.

Buffer and Uint8Array needles are now compared as complete byte sequences.
This changes the previous behavior for one-byte and odd-length needles when a
UTF-16LE/UCS2 encoding argument is supplied.

Byte-wise UTF-16LE searching may affect performance compared to the previous
16-bit implementation. The big-endian decoding path should also be confirmed
by CI.

Testing

ninja -C out/Release node
./tools/test.py -p tap parallel/test-buffer-indexof parallel/test-buffer-includes
./tools/test.py -p tap parallel/test-buffer-generic-methods
make test-doc

[inoway46]

Fix typo (usc2 → ucs2) and wording.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (b9dacd4) to head (d033ea6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64917   +/-   ##
=======================================
  Coverage   90.29%   90.30%           
=======================================
  Files         760      760           
  Lines      247061   247046   -15     
  Branches    46585    46580    -5     
=======================================
- Hits       223092   223088    -4     
+ Misses      15451    15447    -4     
+ Partials     8518     8511    -7     

Files with missing lines	Coverage Δ
src/node_buffer.cc	69.08% <100.00%> (-0.33%)	⬇️

... and 28 files with indirect coverage changes

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