Internal: Implement webpack-dev-server

[christianztamayo]

I'm unable to create an issue, so submitting a PR instead.

This improves productivity with live reload via webpack-dev-server. Helpful for developing plugins, doing customizations and simple as running yarn start

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Additional information

It would be nice to have this on other builds as well

[Reinmar]

Hi!

First of all, sorry for the late reply. December was crazy for us.

I think that your change makes sense. I don't know how many developers will actually use it but it's not a complex thing, so it shouldn't cause many maintenance issues.

One thing that we definitely need is a documentation. Currently the docs for installing plugins and creating custom builds are really messy (due to some historical reasons) so it's not easy to tell where this information should land. But
perhaps somewhere around this place would be fine https://ckeditor.com/docs/ckeditor5/latest/builds/guides/development/custom-builds.html#rebuilding-the-bundle.

Other than that – as you said – we need to copy it to other builds too.

If you could add the documentation in github.com/ckeditor/ckeditor5 I can then copy your change to other builds.

[christianztamayo]

Apologies for the late response. Sure, I'll publish a PR on the ckeditor5 repo shortly.

[christianztamayo]

Removed the host and port for devServer config and just default to http://localhost:8080/

Documentation PR now also available at ckeditor/ckeditor5#6064

[Reinmar]

Thanks! I tested your PR and it works. The docs look good to me too.

One thing I'm unsure though is whether it's good that this commands clears out the build/ directory. Maybe it'd be better if it used a different directory for its job? WDYT?