Ensure we process Tailwind CSS features when using @reference

[RobinMalfait]

This PR fixes an issue where if you only used @reference that we didn't process Tailwind CSS features.

We have a 'quick bail check', in the PostCSS plugin to quickly bail if we konw that we don't need to handle any Tailwind CSS features. This is useful in Next.js applications where every single CSS file will be passed to the PostCSS plugin.

If you use custom font ins Next.js, each of those fonts will have a CSS file as well.

Before we introduced @reference, we used @import "tailwindcss" reference, which passed the bail check because @import was being used. Now we have @reference which wasn't included in the list.

This is now solved.

Fixes: #16056

Test plan

Added a failing test that is now failing after the fix.

[philipp-spiess]

Needs to also handle @variant I think? You can use it with no config for the static variants.

[RobinMalfait]

Needs to also handle @variant I think? You can use it with no config for the static variants.

Ah yep good catch. Added a test and a fix.