feat: changeset

yippie, my first ever changeset in the Starlight package 🎉

Author: trueberryless

.changeset/eight-masks-kick.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Adds validation for user config routeMiddleware so it does not conflict with [Astro's middleware](https://docs.astro.build/en/guides/middleware/).