Support @import "tailwindcss" using top-level index.css file

[RobinMalfait]

This PR adds an index.css file to the root of the package that contains the @tailwind directives for each of Tailwind's layers.

/* node_modules/tailwindcss/index.css */
@tailwind base;
@tailwind components;
@tailwind utilities;

If you have postcss-import configured in your project, this makes it possible to add Tailwind to your CSS using a simple @import instead of writing out the @tailwind directives yourself:

@import "tailwindcss";