How to document which editor interfaces are required by specific features?

[Reinmar]

This issue is based on #302.

The ckeditor5-core package is just a framework for creating RTEs. Final implementations, called also creators, are provided by other packages. Those creators extend base classes (atm it's Editor and StandardEditor) exposed by the core.

Now, on each level (Editor -> StandardEditor -> ClassicEditor) we've got a different set of properties available. E.g. Editor doesn't have keystrokes property because it's UI related. It appears in StandardEditor which is understood as "browser based editor with a single editable".

Most features are split into their "engine" and "full" parts. The BoldEngine provides only the command, schema and converters. It's compatible with editors which have document and commands and editing properties, so by Editor's children (there's a trick – Editor's constructor doesn't set editing property but defines it and says that it must be set). However, Bold requires much more – keystrokes and ui. It will work with StandardEditor and every other editor implementing the same interface.

What we're missing are definitions of these interfaces and features defining what kind of interface they need. Without them, end-developers won't know what kind of creator is needed for the set of features that he/she is using.

The easiest solution will be to create a set of standard interface definitions and in each feature's documentation define which interface must be provided.

We can also automate this by introducing new properties. E.g. Editor could have a implements property which is an array of strings (interface names) and feature requiresEditor (or something more intuitive) which defines which interfaces the editor must fulfill. If requirements are not met, we could throw an error.

[Reinmar]

Related ticket: #327.