gm, invariant, object-assign, react-file-reader-input, eslint-plugin-promise, postcss-import were unused dependencies
eslint-plugin-standard is not needed anymore with latest eslint-config-standard
babel-core is replaced by @babel/core
babel-eslint is replaced by @babel/eslint-parser
babel-preset-env is replaced by @babel/preset-env
babel-preset-react is replaced by @babel/preset-react
babel-plugin-transform-class-properties is included as part of @babel/preset-env
terser-webpack-plugin-legacy removed, terser-webpack-plugin is a dependency of webpack 5, and enabled automatically when using mode: production.

I rebased and push forced to add the fixes from #648
If you want to test, I pushed the build in my code-cleanup-plus-react-update-with-dist branch, it's the same commits as code-cleanup-plus-react-update but with an additional commit to push the build

<a-scene inspector="url:https://cdn.jsdelivr.net/gh/vincentfretin/aframe-inspector@f89a0b44190b1591d2d82f6f47ed7eecf4ef9b38/dist/aframe-inspector.min.js"

Copy html to clipboard and opening the textures modal doesn't work. I'll look into it later.

<a-scene inspector="url:https://cdn.jsdelivr.net/gh/vincentfretin/aframe-inspector@96b9ec69b4f1b5579cfd8d06d4b7971df64ace3a/dist/aframe-inspector.min.js"

includes the fix for #638 (comment)

Now using GLTFExporter directly from three package instead of the old one in vendor folder.

This needs rebase

Is it possible to break this down in smaller PRs?

Now that all the other PRs are merged, this one contains only the big update of react and all dependencies. I could split the last commit in its own PR where I add husky but currently it depends on another commit that changes package-lock.json, that would be a headache to merge this commit first then rebase this PR. I can separate it when I'm done fixing the regression with the clipboard.

38k lines the package-lock.json file? Is not that a bit crazy 😄? Can it be purged / simplified? Adds tons of noise

Ah ah no it can't be simplified, we need to commit the file as is.
It's just all the dependencies that are installed in a typical react app. The file is needed to install exactly those versions when you run npm install.
The package-lock.json in aframe repo is 25k lines. :)

Thanks for merging, I'll fix the two remaining todos #662 asap.