Polyfill for CSS Mixins level 1, `@function` rule.

### What would you want to propose?

Spec: https://www.w3.org/TR/css-mixins-1/
This one would be pretty useful and should be easy to polyfill.
Currently available in Chromium for testing.

### Suggested solution

To implement this plugin one has to do something as follows:
Inline function contents, mangle local variable names, remember to handle inline media queries and nested function calls.

### Additional context

Some examples:
https://una.im/5-css-functions/

With regards to how it resolves variables it is slightly weird.
Spec is in the early stages, I'm sure it will change in the future.

### 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

Beginne mit der CSS Mixins Level 1 specification und den verlinkten CSS functions examples, um das vorgeschlagene Verhalten zu verstehen. Begrenze den Plugin-Umfang auf das Inlining von Funktionsinhalten, das Name Mangling lokaler Variablen, Inline-Media-Queries, verschachtelte Funktionsaufrufe und die variable-resolution rules der specification. Als abgeschlossen gilt die Funktion, wenn sie trotz der frühen Entwicklungsphase der specification implementiert ist und ihr Verhalten durch geeignete Beispiele oder Tests abgedeckt wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.