fix: tailwind-syntax should allow plugin atrule

[tisonkun]

For example:

@import "tailwindcss";
@plugin '@tailwindcss/typography';

[linux-foundation-easycla]

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[tisonkun]

@lumirlumir The failure seems to be unrelated to this patch.

[lumirlumir]

@tisonkun Yes, I’ve re-run the CI 👍

[nzakas]

We are going to move Tailwind support to a separate package. See #166