CSS Custom Functions and Mixins

### What would you want to propose? Spec: https://drafts.csswg.org/css-mixins/ The time has come. SCSS mixins are coming to CSS. Currently, this is hidden behind a feature flag in Chrome. We need a PostCSS preprocessor for it. ### Suggested solution Similar to https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-custom-media, I would recommend to create a `postcss-custom-functions` package that adds the functionality. In the future, the (polyfil at build time) could be removed. However, that will certainly take several years. ```css @function --shadow(--shadow-color <color> : inherit) { /* If --shadow-color argument isn't passed, or doesn't successfully parse as a <color>, try to use the --shadow-color *property* from the element instead */ /* var(--shadow-color) refers to the --shadow-color parameter, rather than a custom property, but can still use a fallback value as normal */ result: 2px 2px var(--shadow-color, black); } .foo { --shadow-color: blue; box-shadow: --shadow(); /* produces a blue shadow */ /* or just */ box-shadow: --shadow(blue); } ``` ### Additional context _No response_ ### Validations - [x] Follow our [Code of Conduct](https://github.com/csstools/postcss-plugins/blob/master/CODE_OF_CONDUCT.md) - [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate. ### Would you like to open a PR for this feature? - [ ] I'm willing to open a PR