Add mbstring support for GB18030-2022 text encoding

The previous version of the GB-18030 standard was published in 2005.
This commit adds support for the updated (2022) version of this t

Add mbstring support for GB18030-2022 text encoding

The previous version of the GB-18030 standard was published in 2005.
This commit adds support for the updated (2022) version of this text
encoding. The existing GB18030 implementation has been left unchanged
for backwards compatibility; users who want to use the new standard
must explicitly indicate the desired text encoding is 'GB18030-2022'.

The document which defines GB18030-2022, published by the government
of the People's Republic of China, defines three levels of standards
compliance. This implementation is intended to achieve Implementation
Level 3, which is the highest level of compliance.

Experts in the GB18030 standard are requested to assess this
implementation and report any deviation from the standard.

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Combine CJK encoding conversion code in a single source file

This will make it easier to combine duplicated code between all the
CJK text encodings (a significant amount is already combi

Combine CJK encoding conversion code in a single source file

This will make it easier to combine duplicated code between all the
CJK text encodings (a significant amount is already combined in this
commit, such as the repeated definitions of SJIS_DECODE and
SJIS_ENCODE), but I hope to remove even more redundancy in the future.

The table used to implement mb_strlen for CP932 has been changed to
the same table as "SJIS-win".

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