Port shadcn/typeset

Port the new shadcn/typeset feature from shadcn-ui/ui#11133 to shadcn-svelte.

Scope:

• add the Typeset stylesheet and Svelte builder
• integrate Typeset with the docs renderer
• add the Typeset guide and changelog entry
• verify visual parity against the upstream builder

Upstream: https://github.com/shadcn-ui/ui/pull/11133