Support directory imports for index.{ts,cts,mts}

[thecrypticace]

Fixes #1196

[RobinMalfait]

Do we need something similar for Tailwind CSS itself? 🤔

[thecrypticace]

I think it'll already work in Vite (but reported it as working in Tailwind CSS) but maybe do need to do that in @tailwindcss/node for postcss and the CLI. Lemme check…

[thecrypticace]

From @tailwindcss/node and @tailwindcss/cli:

So not exactly but we should probably add .mts and .cts to the lists there.