This adds a script to generate the mapping files for Traditional Chinese Big-5-based codecs, as discussed in the issue.

I initially planned to add support for later versions of HKSCS, but I decided to keep this minimal so to close gh-84508. The topic of refreshing the mappings is split to its own issue in gh-93271.

So this generates mappings_tw.h and mappings_hk.h files identical to the existing versions (with only one new line difference).

Notes about the mapping files:

• I did not include the BIG5.TXT and CP950.TXT files that are available on the Unicode website. It looks like these are available for redistribution, so I can add them to the PR if needed.
• I also did not include the hkscs-2004-big5-iso.txt file, but that's a different story. The terms of use from the source website include a clause that unilaterally binds to any update to the terms (clause 2), which I believe is incompatible with redistribution as part of CPython.