[docs-infra] Remove all ES6 transpilation features

[oliviertassinari]

Summary

The docs transform modern JavaScript features for old browsers, but it's not necessary.

https://pagespeed.web.dev/analysis?url=https%3A%2F%2Fmaster--material-ui.netlify.app%2F

Lighthouse uses Cannot call a class as a function to detect https://babeljs.io/docs/babel-plugin-transform-classes https://github.com/GoogleChrome/lighthouse/blob/369979f498bd6560127e10476edffb264d4fa3b9/core/audits/byte-efficiency/legacy-javascript.js#L301C22-L301C55.

Seeing this on https://master--material-ui.netlify.app/ is wrong:

We should at minimum go into loose mode: https://github.com/vercel/next.js/blob/5ff2731c589692ed86379f876a38e1ca46f5761e/packages/next/src/build/babel/preset.ts#L161.

Examples

As you can see from https://codesandbox.io/p/sandbox/loving-goodall-f5p6nl?file=%2Fsrc%2Fcomponents%2FApp.js%3A13%2C26 the loose mode already has a good bundle size reduction impact.

Motivation

A better docs experience. Upstream dependency with Next.js vercel/next.js#65540. Moving to SWC could be enough otherwise.

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