Extended PostCSS support

[luksak]

Currently PostCSS is treated as CSS. We could provide better integration for PostCSS by supporting syntax of the most common PostCSS plugins. Some features missing are postcss-mixins and postcss-custom-media. Currently zed doesn't understand them, treats them as unknown at-rules and can't provide code completion for them:

This is where PostCSS is declared to be treated the same as CSS:

https://github.com/zed-industries/zed/blob/41ac6a87641b7d2a7c2b1261ebe5fe06e82a2c40/crates/languages/src/css/config.toml#L1C1-L3C43

This is the most popular PostCSS extension for VSCode providing language server: https://github.com/csstools/postcss-language But it has only very rudimentary support for PostCSS plugins.

This VSCode extension provides better support (such as postcss-mixins) and also provides a language server: https://github.com/vunguyentuan/vscode-postcss

Could we get a PostCSS extension for zed that provides one of those language servers?