Support @import "tailwindcss" using top-level index.css file (#11205)

* make `@import "tailwindcss"` work out of the box

* update changelog

Author: RobinMalfait

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - [Oxide] Use `lightningcss` for nesting and vendor prefixes in PostCSS plugin ([#10399](https://github.com/tailwindlabs/tailwindcss/pull/10399))
+- Support `@import "tailwindcss"` using top-level `index.css` file ([#11205](https://github.com/tailwindlabs/tailwindcss/pull/11205))
 
 ### Changed
 

index.css

@@ -0,0 +1,5 @@
+@tailwind base;
+
+@tailwind components;
+
+@tailwind utilities;