Add system-ui generic family

The generic family of 'system-ui' name is being proposed in a draft of next CSS Fonts.
This would be nice to support in fontconfig too.

https://www.w3.org/TR/css-fonts-4/
2 jobs for system-ui in 6 minutes and 5 seconds
Status Job ID Name Coverage
  Build
passed #142453
shared-build

00:06:05

passed #142454
static-build

00:04:48