[Fix] `webpack@5.0.0-beta.30`: Invalid configuration (#17045)
[Fix] `webpack@5.0.0-beta.30`: Invalid configuration (#17045)

Earlier today #17038 was merged which I opened to fix a problem when using `webpack@5.0.0-beta.30` with Next.js using the new Webpack 5 support. In that PR, the only change was the renaming of a configuration key. I later discovered that the change on the Webpack side was different than I initially thought, and this meant that the fix I submittted to Next.js didn't work.

This PR intends to fix the remaining problems. Webpack 5 now accepts a `environment` key that can be used to configure the target output. Previously, this was known as `ecmaVersion` and accepted a number. Now, `environment` accepts a configuration object with individual options. I've configured this in such a way where it resembles an ES5 environment:
```js
environment: {
   arrowFunction: false,
   bigIntLiteral: false,
   const: false,
   destructuring: false,
   dynamicImport: false,
   forOf: false,
   module: false,
}
```
zeit/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,flybayer/next.js,azukaru/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,zeit/next.js