fix: allow user to add Vitepress configuration to ESM packages

If users have a `type: module` entry in their `package.json`, they
could not use VitePress with a `config.js` file.

This change makes it so that `config.cjs` is searched before
`config.js`.  Node's 'require' function allows files with such
extensions to be loaded even from within an ES module project.

* docs/guide/configuration.md: Mention config.cjs

* src/node/config.ts: Consider config.cjs

docs/guide/configuration.md

@@ -20,4 +20,9 @@ module.exports = {
 }
 ```
 
+::: warning
+If your project uses [ES modules](https://nodejs.org/api/esm.html),
+name this file `.vitepress/config.cjs` instead.
+:::
+
 Check out the [Config Reference](/config/basics) for a full list of options.

src/node/config.ts

@@ -111,20 +111,19 @@ export async function resolveConfig(
   return config
 }
 
-export async function resolveUserConfig(root: string) {
+export async function resolveUserConfig(root: string): Promise<UserConfig> {
+  for (const name of ['config.cjs', 'config.js']) {
     // load user config
-  const configPath = resolve(root, 'config.js')
-  const hasUserConfig = await fs.pathExists(configPath)
+    const configPath = resolve(root, name)
     // always delete cache first before loading config
     delete require.cache[configPath]
-  const userConfig: UserConfig = hasUserConfig ? require(configPath) : {}
-  if (hasUserConfig) {
+    if (await fs.pathExists(configPath)) {
       debug(`loaded config at ${chalk.yellow(configPath)}`)
-  } else {
-    debug(`no config file found.`)
+      return require(configPath)
     }
-
-  return userConfig
+  }
+  debug(`no config file found.`)
+  return {}
 }
 
 export async function resolveSiteData(