[Generic font families] Add Kyokasho-tai as a generic font family

[kidayasuo]

Commenting on https://github.com/w3c/i18n-discuss/wiki/Generic-font-families, via JLReq TF meeting notes on 2023-12-19.

Kyokasho-tai (or textbook style) is one of well established and popular typeface in Japan. It originally derived from kai (written by calligraphy brush), but recent designs are shifting towards styles that are written by a pen. Kyokasho style has distinct shape that is suitable for children to learn how to write. The shape of the Kanji follows the Ministry of Education's Elementary School Kanji Learning Guideline.

The most important use is text books especially for younger children. Also it is used in books to give readers somewhat nostalgic impressions. Both Windows and Apple platforms have this typeface.

One caveat about the use of this typeface is that it has its own Latin/Numeric characters and they should be used to meet expectations. That is they should not be replaced by different Latin typefaces in fallback, even if they are in textbook styles in American or European contexts.