[WIP] Webpack 5 upgrade

[igmoweb]

Work in Progress

• Upgrade to Webpack 5.0 and all its dependencies and loaders
• Introduce new postcss dependency as it's needed for the postCSS loader from now on
• Update PostCSS loader syntax
• Update new Webpack manifest plugin syntax
• Switch deprecated option

Missing:

• Production build doesn't work yet due to some obscure error in Mini Extract CSS Plugin

Helpers aren't working with webpack 5 and I had to upgrade all the packages (we may not need all those upgrades, though, but it was the most straightforward way for now.

There's also a consideration to have in mind: This will break any project setup that doesn't bump its webpack version so we may want to add some notification

[kadamwhite]

This is now outdated vs main following the merge of several PRs that included some of these loader changes. We can target Webpack 5 support for a v1.0 target, I will push out v0.10 in the interim with continued Webpack 4 support (using updated loaders)