[2.0.0-beta4] Add a stories specific discovery

Problem/Motivation

In #3360373-8: [2.0.0-beta2] Allow multiple previews for each pattern, Florent asked:

I think it would be nice to have a mechanism which allows a module or theme to add stories to an existing component without having to redeclare the component.

Proposed resolution

Do we introduce a component story plugin type?

So, we can implement discoveries for :

• UI Patterns 2 stories, based on Render arrays serialized in YML. We move them to separate files (as initially asked by Duaelfr)? One story by file? It will make compoennt definitions YML smaller and allow to declare additional stories as asked by Florent.
• JSON schema examples, as described in #3414774: JSON schema examples property
• Former Mateu's CL JSON format
• New Mateu's Twig format (this one looks complicated to parse)