gh-139156: Use PyBytesWriter in UTF-16 encoder

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Replace PyBytes_FromStringAndSize() and _PyBytes_Resize() with the PyBytesWriter API.

• Issue: Use PyBytesWriter in Unicode codecs #139156

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Benchmark:

import pyperf
runner = pyperf.Runner()
for size in (3, 100, 1000):
    runner.timeit(f'{size:,} ASCII chars',
        setup=f's="x"*{size}',
        stmt='s.encode("utf16")')
for size in (3, 100, 1000):
    runner.timeit(f'{size:,} UCS-4 chars',
        setup=f's=chr(0x10ffff) * {size}',
        stmt='s.encode("utf16")')

Results with CPU isolation and python -m pyperf system tune:

Benchmark	ref	pep782
3 ASCII chars	91.2 ns	93.0 ns: 1.02x slower
3 UCS-4 chars	101 ns	116 ns: 1.14x slower
100 UCS-4 chars	402 ns	416 ns: 1.04x slower
1,000 UCS-4 chars	2.83 us	3.03 us: 1.07x slower
Geometric mean	(ref)	1.04x slower

Benchmark hidden because not significant (2): 100 ASCII chars, 1,000 ASCII chars

UPDATE: Results recomputed with python -m pyperf system tune.