[Feature]: add `$schema` field for registry items

[stabldev]

For generated component json files, add a $schema field for json lsp support.
Take inspiration from:

• https://github.com/shadcn-ui/ui/tree/main/apps/v4/public/schema
• https://github.com/headcn/ui/blob/main/packages/headcn/src/registry/schema.ts