fix: overload defineConfig

revert #381 & daaac8d

Author: sxzz

src/config.ts

@@ -3,9 +3,11 @@ import type { UserConfig, UserConfigFn } from './options'
 /**
  * Defines the configuration for tsdown.
  */
-export function defineConfig<const T extends UserConfig | UserConfigFn>(
-  options: T,
-): T {
+export function defineConfig(options: UserConfig): UserConfig
+export function defineConfig(options: UserConfigFn): UserConfigFn
+export function defineConfig(
+  options: UserConfig | UserConfigFn,
+): UserConfig | UserConfigFn {
   return options
 }