fix: windicss markdown parsing

[Tahul]

Span Syntax support using this [Span]{.bg-blue-500} syntax.

Seems like we need a custom extractor for that, as @antfu mentionned it.

Here is an example of implementation from Slidev for reference:
https://github.com/slidevjs/slidev/blob/31e3a505df22a4294b0d3d669bbacd5ce9a6a75c/packages/client/windi.config.ts#L22-L40