fix: fix webpack 5 config for live demo

[Vayel]

Fix #494

Test

1. npm run build-demo
2. Open packages/zmarkdown/public/index.html

[StaloneLab]

Thanks a lot for your work, I cannot understand anything to Webpack, but I tested it locally and the live demo is working again!

I left two minor comments to be addressed, and I'll merge your changes right after.

[StaloneLab]

There are no more linting problems, so apart from the README part, to which I answered, this is ready to be merged.

Unit testing is still failing because of package-lock.json, but no need to worry about this, I'll fix it once merged, as it can prove quite tricky not to mess up since we use a monorepo.

[StaloneLab]

Ok, I thought you wanted to add something to the README but let's merge as is. I am ready to merge other changes you suggested if you want to work on them, such as:

• adding a link to the package/zmd README in the main one;
• adding some developer documentation in the README.

You might want to do this last one while working on something else, if you want to involve in the project.

[StaloneLab]

Live demo is indeed working: https://zestedesavoir.github.io/zmarkdown/. Thanks again for your work!