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]

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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.

[jonsam-ng]

I'm really curious if this feature is still available and I'm looking forward to it.

[constantineMelios85]

Great addition, element.slots with children staying as the default slot is the right call: named slots stay purely additive, so existing specs and anything rendering children keep working with zero migration.

We're evaluating named slots for our own app built on json-render, and this shape is exactly what we'd want to align to.
One gap for adopters: it's React-only (packages/react) — react-native has the same flatten-children behavior and would need the same treatment for parity. A follow-up issue tracking it would help.

Would love to see this merged, the declaration side has been in the catalog since v0.4.0, and this finally closes the rendering half.