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

Start with the CSS Mixins Level 1 specification and the linked CSS functions examples to understand the proposed behavior. Scope the plugin around inlining function contents, local-variable name mangling, inline media queries, nested function calls, and the specification's variable-resolution rules. Done means the feature is implemented despite the early-stage specification and its behavior is covered by suitable examples or tests.

Scritto dal modello di indicizzazione a partire dal testo della issue.