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

• Follow our Code of Conduct
• 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

Comienza con la CSS Mixins Level 1 specification y los CSS functions examples enlazados para entender el comportamiento propuesto. Limita el alcance del plugin a la inclusión directa del contenido de las funciones, el name mangling de los nombres de variables locales, las consultas de medios inline, las llamadas a funciones anidadas y las reglas de resolución de variables de la specification. Se considera terminado cuando la funcionalidad está implementada a pesar de la etapa inicial de la specification y su comportamiento está cubierto por ejemplos o pruebas adecuados.