Direct support for @import 'tailwindcss/{layer}' syntax

[adamwathan]

This PR adds support for writing your layers this way:

@import "tailwindcss/base";
@import "tailwindcss/components";
@import "tailwindcss/utilities";

This already works if you are using postcss-import or similar, but this makes it work even if you aren't, which makes it a lot easier to document things. Tailwind already supports this on the main branch, bringing this in here as it came up in #143.