`@import` rules should be hoisted to the top of the generated CSS file

[menelaos]

Steps to reproduce:

1. stack new myapp yesodweb/simple
2. Add @import "https://unpkg.com/open-props"; to the top of templates/default-layout.lucius
3. In the same file, change the background-color for .masthead from rgb(27, 28, 29) to var(--red-5)
4. Start the development server
5. Observe that the background color is not red

Background:

If we open the dev tools, we see that --red-5 is not set:

The problem seems to be that in the generated CSS file, the @import rule is not at the top of the file and thus gets ignored:

While I haven't tested it, I presume this affects other at-rules, too.