fix: narrow defineConfig return type (#12021)

Author: fi3ework

packages/vite/src/node/config.ts

@@ -104,6 +104,9 @@ export type UserConfigExport = UserConfig | Promise<UserConfig> | UserConfigFn
  * The function receives a {@link ConfigEnv} object that exposes two properties:
  * `command` (either `'build'` or `'serve'`), and `mode`.
  */
+export function defineConfig(config: UserConfig): UserConfig
+export function defineConfig(config: Promise<UserConfig>): Promise<UserConfig>
+export function defineConfig(config: UserConfigFn): UserConfigFn
 export function defineConfig(config: UserConfigExport): UserConfigExport {
   return config
 }