Allow theme(…) options when using @import

[RobinMalfait]

This PR improves the developer experience when trying to use theme(…) options on an import.

Today, if you want to use Tailwind CSS, you can import it as:

@import "tailwindcss";

But if you want to use any of the theme(…) options, like the static theme option, then you had to use this instead:

@layer theme, base, components, utilities;

@import 'tailwindcss/theme' layer(theme) theme(static);
@import 'tailwindcss/preflight' layer(base);
@import 'tailwindcss/utilities' layer(utilities);

In this scenario you have to be careful, because the layer(…) must be the first option after an import (according to the spec). So if you use @import 'tailwindcss/theme' theme(static) layer(theme); then that's not going to work either.

This PR solves that by allowing you to use the theme(…) options directly on the @import statement:

@import 'tailwindcss' theme(static);

The only edge case is when you want to use theme(reference). A typical use case is for projects with <style> blocks where you want to reference the CSS variables from the theme.

If you use @import 'tailwindcss' theme(reference);, then all @theme blocks will be references and you can reference theme values. This is good. The bad part is that @import 'tailwindcss'; also includes preflight CSS. This means that we will import the preflight CSS for every <style> block. This is probably not what you want.

The solution is to use @reference 'tailwindcss'; instead which strips all of that information and only gives you access to CSS variables.

This PR also makes sure that if you use theme(reference) on an import that we still throw an error and suggest you use @reference instead. This is not a breaking change because right now if you use @import with theme(…) options it will already throw an error.

Test plan:

1. Added dedicated tests to make sure we don't throw anymore.
2. Added test to make sure that we do throw when using theme(reference) on an import.

[philipp-spiess]

Noice