Fix TS2742 error when inferring exported Config type

Change `Config` from a type alias to an interface so TypeScript
preserves its name in declaration emit instead of resolving through
to `UserConfig` in the hashed internal chunk file.

Fixes #19706

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: silverwind

packages/tailwindcss/src/index.ts

@@ -34,7 +34,7 @@ import { segment } from './utils/segment'
 import { topologicalSort } from './utils/topological-sort'
 import { compoundsForSelectors, IS_VALID_VARIANT_NAME, substituteAtVariant } from './variants'
 import { walk, WalkAction } from './walk'
-export type Config = UserConfig
+export interface Config extends UserConfig {}
 
 const IS_VALID_PREFIX = /^[a-z]+$/