update webpack config to webpack5

[timea-solid]

Upon local testing I followed the advice on https://github.com/solid/solid-panes#development
I get errors:

The problem is not in profile-pane which was updated to webpack5. It is because solid-panes webpack config is not updated.

[josephguillaume]

Sorry, I was naughty and also updated the webpack config in #320

The profile-pane on GitHub works fine, i.e. those errors have been fixed but not yet published to npm
I fixed the profile-pane errors locally within a solidos environment by using
npm run add profile-pane
npx lerna bootstrap --force-local

There might have been a build step in there somewhere too.
And obviously I had to manually select the form pane and point it at a test file.
Not sure if there's a way for me to share my test environment.