add @mixin and @function

[anneangersbach]

The CSS Working Group has resolved to adopt this proposal, and begin drafting a new specification for CSS Mixins & Functions.

https://drafts.csswg.org/css-mixins-1/

https://groups.google.com/a/chromium.org/g/blink-dev/c/bvi4D7eD7wI/m/djYVLu6OAwAJ?pli=1

https://css.oddbird.net/sasslike/mixins-functions/

https://www.bram.us/2025/02/09/css-custom-functions-teaser/

[Mouvedia]

duplicate of #7110