gh-139156: Optimize _PyUnicode_EncodeCharmap()

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Specialize _PyUnicode_EncodeCharmap() for EncodingMapType which is used by Python codecs such as iso8859_15.

• Issue: Use PyBytesWriter in Unicode codecs #139156

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Benchmark:

import pyperf
runner = pyperf.Runner()
sizes = (1, 100, 10_000)
for size in sizes:
    runner.timeit(f'{size:,} ASCII chars',
        setup=f's="x"*{size}',
        stmt='s.encode("iso8859-15")')
for size in sizes:
    runner.timeit(f'{size:,} UCS-2 chars',
        setup=f's="€"*{size}',
        stmt='s.encode("iso8859-15")')

Results using the main branch as the reference:

Benchmark	ref	change
1 ASCII chars	629 ns	622 ns: 1.01x faster
100 ASCII chars	1.13 us	997 ns: 1.13x faster
10,000 ASCII chars	49.8 us	31.8 us: 1.57x faster
1 UCS-2 chars	630 ns	622 ns: 1.01x faster
100 UCS-2 chars	1.18 us	1.06 us: 1.12x faster
10,000 UCS-2 chars	54.3 us	38.2 us: 1.42x faster
Geometric mean	(ref)	1.19x faster

Results using Python 3.14 as the reference:

Benchmark	314	change
1 ASCII chars	647 ns	622 ns: 1.04x faster
100 ASCII chars	1.12 us	997 ns: 1.12x faster
10,000 ASCII chars	45.2 us	31.8 us: 1.42x faster
1 UCS-2 chars	648 ns	622 ns: 1.04x faster
100 UCS-2 chars	1.17 us	1.06 us: 1.10x faster
10,000 UCS-2 chars	49.8 us	38.2 us: 1.30x faster
Geometric mean	(ref)	1.16x faster

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cc @serhiy-storchaka