ESM instead of CommonJs

[quriosapien]

Modified the CommonJs style export of config.js to the ESM style export.

Vite is all about advancement of tools. Why not reflect it in documents, by using ESM instead of CommonJs.

[WebMechanic]

I believe this is where the voyage goes like with the rest of the Vueniverse:

// .vitepress/config.js
import {defineConfig} from 'vitepress'
export default defineConfig({ ... });

[yyx990803]

We actually ship defineConfig and defineConfigWithTheme now, both with full type support, which we should document at some point.