fix: webpack 5 polyfilling compatibility

[julrich]

Seemingly something changed with webpack polyfilling, affecting us (assert missing).

This fixes the problem for the Gatsby theme: 466b7fd

Maybe we can prevent this from impacting consuming projects? If so, we should create a ticket for @kickstartDS itself.

More info:

• https://webpack.js.org/blog/2020-10-10-webpack-5-release/#automatic-nodejs-polyfills-removed
• https://webpack.js.org/configuration/resolve/#resolvefallback

[julrich]

One more relevant link: remarkjs/react-markdown#632