feat(studio): preview content elements from skills

[davidvitora]

This feature allows the flow generator to return a list of content elements to be previewed in the flow editor.

Example:

The content element ids are saved in the node definition (same logic as transitions)

It can be specified in the flow generator function in the return object.

[davidvitora]

@laurentlp @EFF @slvnperron @allardy

There is also the possibility of having a "preview" property instead of "previewElements", where we could specify any kind of ActionItem to be previewed, including actions, which we can also preview today at normal nodes.

It would be something like

preview: [{
  type: sdk.NodeActionType.RenderElement,
  name: 'content-element-id-sdxsasf'
}, {
  type: sdk.NodeActionType.RunAction,
  name: 'basic-skills/choice_parse_answer'
}]