fix: port the latest defineConfig type from vite

Ported from vitejs/vite#13792

Fully fixes issues like vuejs/create-vue#317
and vuejs/create-vue#313

For an easy reproduction, please refer to https://github.com/sodatea/viteste-define-config-bug-repro

I didn't include a test in this PR because:

1. The reproduction is a bit complicated, it only fails on the very
latest Vite versions, but updating Vite would mess up the lockfile.
2. By not updating the dependencies, we can make sure that this fix is
compatible with older versions of Vite, too.

packages/vitest/src/config.ts

@@ -10,13 +10,18 @@ export { configDefaults, defaultInclude, defaultExclude, coverageConfigDefaults
 export { mergeConfig } from 'vite'
 
 export type { ConfigEnv, ViteUserConfig as UserConfig }
+export type UserConfigFnObject = (env: ConfigEnv) => ViteUserConfig
+export type UserConfigFnPromise = (env: ConfigEnv) => Promise<ViteUserConfig>
 export type UserConfigFn = (env: ConfigEnv) => ViteUserConfig | Promise<ViteUserConfig>
-export type UserConfigExport = ViteUserConfig | Promise<ViteUserConfig> | UserConfigFn
+export type UserConfigExport = ViteUserConfig | Promise<ViteUserConfig> | UserConfigFnObject | UserConfigFnPromise | UserConfigFn
 
 export type UserProjectConfigFn = (env: ConfigEnv) => UserWorkspaceConfig | Promise<UserWorkspaceConfig>
 export type UserProjectConfigExport = UserWorkspaceConfig | Promise<UserWorkspaceConfig> | UserProjectConfigFn
 
-export function defineConfig(config: UserConfigExport) {
+export function defineConfig(config: ViteUserConfig): ViteUserConfig
+export function defineConfig(config: Promise<ViteUserConfig>): Promise<ViteUserConfig>
+export function defineConfig(config: UserConfigExport): UserConfigExport
+export function defineConfig(config: UserConfigExport): UserConfigExport {
   return config
 }