Move CSS files to root for easier imports

[adamwathan]

This PR moves the source files we use to generate our dist files to the root of the project so that it's a bit more ergonomic to import them using postcss-import.

Previously, you'd need to do this:

@import "tailwindcss/css/preflight";
@import "tailwindcss/css/components";
@import "tailwindcss/css/utilities";

Now you can omit the css directory in the middle:

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

Small change but something about it feels a lot nicer for folks using postcss-import. Feels less like you are reaching deep into the internals of the framework and depending on folder structure that isn't really "public API".