[core] Create shared Next.js baseline config

[oliviertassinari]

The current configuration leads to this bug:

So instead, I have created a function to extend a Next.js configuration file. It's similar to https://github.com/martpie/next-transpile-modules/blob/master/src/next-transpile-modules.js. It was suggested in mui/toolpad#866 (comment). I have used the opportunity to normalize how we use env tokens based on mui/toolpad#809 (comment).

https://deploy-preview-34259--material-ui.netlify.app/

This change should unlock:

• [core] Fix scroll restoration in the docs mui-x#5938
• [core] Fix scroll restoration in the docs toolpad#866

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against 2cbf834

[Janpot]

Would it make sense to avoid passing DEPLOY_ENV to the application code entirely, and instead calculate these options centrally, in the next.js baseline config?

env: {
  NEWSLETTER_SUBSCRIBE_URL: isProduction ? '...' : '...',
  GA_ID: isProduction ? 'UA-106598593-2' : 'UA-106598593-3',
  GA_SAMPLING_RATE: isProduction ? 80 : 100,
  ENABLE_TOOLPAD: isProduction,
  // ...
}

[oliviertassinari]

Would it make sense to avoid passing DEPLOY_ENV to the application code entirely

@Janpot What's the value of this change? I understand the value of this indirection for secrets and constants used multiple times, but I don't get it for the rest. My goal in removing docs/src/config was to centralize the logic as much as possible in the same files.

---

A side note. I would also be in favor of removing docs/src/route. The values feel no different from using the URLs directly.

[Janpot]

What's the value of this change?

It's not critical at all, it would mostly improve maintainability a bit and be more defensive code.

• It creates a natural overview of which features are enabled for which environment. This is especially handy when a new environment has to be defined (e.g. new type of integration tests)

• Injects the environment specific flags through the webpack define plugin. This prevents future maintainers from accidentally creating code that doesn't minify unused configuration away. e.g.

  // Minifies to `const NEWSLETTER_SUBSCRIBE_URL = 'url a'`
  const NEWSLETTER_SUBSCRIBE_URL = process.env.DEPLOY_ENV === 'production' ? 'url a' : 'url b'
  
  // Minifies to 
  //   const isProd = () => true
  //   const NEWSLETTER_SUBSCRIBE_URL = isProd() ? 'url a' : 'url b'
  const isProd = () => process.env.DEPLOY_ENV === 'production'
  const NEWSLETTER_SUBSCRIBE_URL = isProd() ? 'url a' : 'url b'
  
  // Injecting process.env.NEWSLETTER_SUBSCRIBE_URL directly prevents this from ever happening

• Avoids duplicating logic in case a certain flag is used in multiple places in the application. Or when the same flag is used between build time and run time

I would also be in favor of removing docs/src/route. The values feel no different from using the URLs directly.

Yes, it's not critical neither. For me, the main advantage of such a file would be around static analyzability of where certain links are used. Especially when people start defining links like /section-${sections.slug} in one place and /section-intro in others. Such constructs are hard to search for. If you use something like a sectionLink('intro') everywhere that could be easily listed using "find references" in vscode

[Janpot]

It's minor, I'm more than fine for this PR to be merged as is

[oliviertassinari]

It has been a week, I'm merging.