feat: add support for using named slots

[wotnak]

Builds on recently committed support for defining component slots in catalog to add support for actually using named slots in spec generation and rendering.

Updated:

• core system prompt,
• codegen traversal,
• react renderer, schema and types,
• json-render-react skill,
• playground,
• docs.

Resolves #39.

[vercel]

@wotnak is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[wotnak]

Fixed type issues reported by the Vercel bot and rebased on the current main.

[amirilovic]

@wotnak are you still working on this one?

[wotnak]

@amirilovic I think it is ready for a review, but I'm happy to address any feedback.
Looks like it also needs another rebase; I'll do that shortly.