Update CKEditor & remove deprecated webpack integration

[ChristopherChudzicki]

Description/Context

We should:

• Update CKEditor to a newer version, probably the latest
  • Studio uses v35 and community provided typescript definitions. CKEditor has first-class support for Typescript starting in v38, so we should remove the community provided type definitions.
• Stop using @ckeditor/ckeditor5-dev-webpack-plugin, which is deprecated.

Plan/Design

For updating the webpack integration, the CKEditor5 guide Integrating from source using webpack describes what to do. See also implementation in MIT Open.

One issue is that starting in v39, CKEditor includes some small branding. See Managing the “Powered by CKEditor” logo. Here's one way to remove it. Or we could leave it, since Studio is just internal anyways.