[Generic font families] Add "rounded (sans-serif)" as a generic font family

[kidayasuo]

Commenting on https://github.com/w3c/i18n-discuss/wiki/Generic-font-families

Maru-Gothic or rounded sans-serif is one of well established and popular typeface categories in Japan. I see ui-rounded in CSS Font 4 but not generic "rounded". I believe the rounded version of sans-serif is pretty generic across different scripts.

Apple's platforms have the rounded sans-serif. However Windows does not have the typeface, the Office has it, i.e. effectively most if not all Windows users in Japan has it.

[xfq]

Simplified and Traditional Chinese also have rounded typefaces, like:

• Yuanti SC/TC (macOS)
• SimYou (Windows)

See also w3c/csswg-drafts#4605