expose as postcss plugin

[danielroe]

For platforms supporting postcss, it would be reasonable to benefit from postcss parsing rather than the current regexp based solution (which could remain fallback)

[lihbr]

Just referencing some related usage.

I tried using fontaine within a Vite + PostCSS + PostCSS Import setup. The Vite plugin is not able on its own to follow import rules for now (from my own understanding of the assets being transformed directly through Vite this way, or maybe I'm doing something terribly wrong), preventing the plugin from detecting fonts as the only file detected ends up looking like the following.

@import "./abstract/fonts.css";
@import "./abstract/default.css";
@import "./abstract/typography.css";
@import "./abstract/prism.css";
@import "./abstract/theme.css";

Thanks for making fontaine 🙏