docs: update "usage" to add needed import (#209)

Co-authored-by: dominikg <[email protected]>

Author: tjwds

README.md

@@ -16,6 +16,7 @@ npm install --save-dev @sveltejs/vite-plugin-svelte
 
 ```js
 // vite.config.js
+import { defineConfig } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
 
 export default defineConfig({

packages/vite-plugin-svelte/README.md

@@ -6,6 +6,7 @@ The official [Svelte](https://svelte.dev) plugin for [Vite](https://vitejs.dev).
 
 ```js
 // vite.config.js
+import { defineConfig } from 'vite';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
 
 export default defineConfig({