refactor: add new overload to the type of defineConfig (#13958)

Author: vinassefranche

packages/vite/src/node/config.ts

@@ -119,6 +119,7 @@ export type UserConfigExport =
  */
 export function defineConfig(config: UserConfig): UserConfig
 export function defineConfig(config: Promise<UserConfig>): Promise<UserConfig>
+export function defineConfig(config: UserConfigFnObject): UserConfigFnObject
 export function defineConfig(config: UserConfigExport): UserConfigExport
 export function defineConfig(config: UserConfigExport): UserConfigExport {
   return config