chore: import both mergeConfig and defineConfig from vitest/config

With the typing fix in Vitest 0.34, it is again possible to use
`defineConfig` and `mergeConfig` from the `vitest` package.

template/config/vitest/vitest.config.js

@@ -1,6 +1,5 @@
 import { fileURLToPath } from 'node:url'
-import { mergeConfig, defineConfig } from 'vite'
-import { configDefaults } from 'vitest/config'
+import { mergeConfig, defineConfig, configDefaults } from 'vitest/config'
 import viteConfig from './vite.config'
 
 export default mergeConfig(