feat: add ClipSet to schema

[adgad]

This is a draft PR for what I think the Clips Schema should be (based on earlier work #86 and what is currently in Workarounds).

There's a few considerations / changes to how it currently works:

1. Rename dataLayout to layoutWidth for consistency with other nodes
   a. ClipSet only allows a limited number of layouts, I assume that is intentional?
2. There is a challenge around transcripts, which is currently modelled as a nested body. In the current component in cp-content-pipeline-ui, it is expecting another RichText graphql type (which has a graphql-ish data structure with fields like raw, structured, references). I'm not really sure how we model that in content-tree, or if. If we need content-tree to be different to cp-content-pipeline (i.e. maintain a workaround), that would also mean the UI component itself isn't really transferable.
3. Similarly, the transcript currently is a with only text. Our current Body node in content-tree doesn't allow Text as a top level node - should it? Or should it be a separate thing?
   a. note - live events also do this, but i've been ignoring them for now!