Docs for MPA config incorrectly suggest using __dirname

[SteveALee]

Describe the bug

The docs for MPA config suggets using __dirname in the config file.

However, the config file is an ES module in my repo created from a template (can't recall exactly how). __dirname is not available in ES Modules, resulting in the following stack strace

failed to load config from /home/wsl/musical-parameters/vite.config.js
error during build:
ReferenceError: __dirname is not defined in ES module scope
This file is being treated as an ES module because it has a '.js' file extension and '/home/wsl/musical-parameters/package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.
    at file:///home/wsl/musical-parameters/vite.config.js?t=1658086233703:13:23
    at ModuleJob.run (node:internal/modules/esm/module_job:198:25)
    at async Promise.all (index 0)
    at async ESMLoader.import (node:internal/modules/esm/loader:385:24)
    at async loadConfigFromFile (file:///home/wsl/musical-parameters/node_modules/vite/dist/node/chunks/dep-561c5231.js:62653:31)
    at async resolveConfig (file:///home/wsl/musical-parameters/node_modules/vite/dist/node/chunks/dep-561c5231.js:62281:28)
    at async doBuild (file:///home/wsl/musical-parameters/node_modules/vite/dist/node/chunks/dep-561c5231.js:43272:20)
    at async build (file:///home/wsl/musical-parameters/node_modules/vite/dist/node/chunks/dep-561c5231.js:43261:16)
    at async CAC.<anonymous> (file:///home/wsl/musical-parameters/node_modules/vite/dist/node/cli.js:747:9)

#6899 has a suitable but it dos not work

import { dirname } from 'node:path'
import { fileURLToPath } from 'url';
const _filename = fileURLToPath(import.meta.url);
const _dirname = dirname(_filename);

I've not provided a PR in case there is more going on.

Reproduction

N/A

System Info

N/A

Used Package Manager

npm

Logs

No response

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[bluwy]

Fixed in #9121. Vite 3.0.1.