gh-152997: Support system locale encodings via an iconv-based codec (GH-153001)

gh-152997: Support system locale encodings via an iconv-based codec (GH-153001)

Where the C library provides iconv(), the codecs module now exposes every
encoding iconv() knows but Python has no built-in codec for -- the POSIX
counterpart of the Windows code-page support.  Use it by name (e.g.
"cp1133"), or with an "iconv:" prefix to force it over a built-in codec.

The codec is a last-resort search function and never shadows a built-in.
Both directions pivot through native-endian UTF-32, keeping one input unit
per code point so error handlers get the exact string position.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>