Migrate demos with external plugins to new installation methods

[f1ames]

This PR migrates demos with 3rd-party plugins to NIM. Follow-up of #38:

I can prepare separate branch migrating all demos with external plugins which will be waiting to be merged. So we have still all demos fully functional (and fully featured) on master and then will be ready when those external plugins are migrated.

It's based on new-installation-methods-v2 branch (#38 PR) and I followed all the same conventions and assumptions. You can read #38 description for all details.

The main difference here is that 3rd-party plugins (WProofreader and Mathtype) are commented out in CKEditor 5 plugins list in it's config in each demo. This way demos can be run and after plugins are migrated to NIM, it should be enough to just uncomment those lines.

[Reinmar]

I've just tripped on this. I provided the license key, as it asked me to, but then surprise, surprise: those premium buttons weren't there. I had quite a big WTH moment :D 

The good thing about our toolbar initialization is that even if you used a button that wasn't registered, the editor will work and you will get a warning.

So it's not necessary to comment out missing buttons. And it's definitely confusing :P

[f1ames]

Good point 👍 I'll fix it - unless you already did 👀

[f1ames]

Why did you rewrite the demos to TS?

My worry here: So far, we keep our resources in JS (docs and most other samples I can think of).

It was a part of this task as we initially discussed with @Witoso and @arkflpc. But I don't know if there is a longer story behind this, maybe they can give more context.

8f66b1f is a bit ugly commit but I tried to figure out why the demos don't work

Turned out there were two reasons: dup modules because of WSC and the change in CSS file names that we did recently.

Yeah, I pushed it before the CSS file names changes, also @pszczesniak pointed that out in the first PR - #38 (review). I see you already updated it 🙇 👍

I started pushing here changes that we have to do to make the cke5-demos repo follow changes that were implemented on ckeditor.com.

@Reinmar maybe let me know what you are working on here, and how cna I help so we don't duplicate the work.

[f1ames]

Ready for review.

The only thing missing is enabling MathType and WProofReader in couple of demos, but this needs to wait for NIM compatible versions of those plugins.

[pszczesniak]

LGTM 👍

[f1ames]

Ready to merge, but I suggest to wait till MathType and WProofReader are bumped to NIM compatible versions?

[pszczesniak]

@f1ames i've tested WProofreader with alpha version "@webspellchecker/wproofreader-ckeditor5": "3.0.0-alpha.0" and it works ok.