fix: work around vitest mergeConfig/defineConfig type issues

Until vitest-dev/vitest#3804 is merged and
released, we can work around the issue by using the exports from `vite`,
they are identical anyway.

In later versions, we should import both `mergeConfig` and
`defineConfig` from `vitest`, though, to better align with their
official documentation.

Fixes #313
Fixes #317

template/config/vitest/vitest.config.js

@@ -1,6 +1,6 @@
 import { fileURLToPath } from 'node:url'
-import { mergeConfig } from 'vite'
-import { configDefaults, defineConfig } from 'vitest/config'
+import { mergeConfig, defineConfig } from 'vite'
+import { configDefaults } from 'vitest/config'
 import viteConfig from './vite.config'
 
 export default mergeConfig(