API changes

What each v11 API is in v12. The codemod rewrites the ones with one v12 form and lists the rest.

v11	v12
`@trpc/server`, `@trpc/client`	`@trpc12/trpc/server`, `@trpc12/trpc/client`
`initTRPC.context<Ctx>().meta<Meta>().create()`	`initTrpc<{ ctx: Ctx; meta: Meta }>()`
`t.router({ … })`, `t.mergeRouters(a, b)`	a plain object, `{ ...a, ...b }` (`router()` and `mergeRouters()` still exist)
`throw new TRPCError({ code })`	`return error({ code })`, or `throw new TrpcError({ code })`
`create({ errorFormatter })`	declared errors (`.errors()`, `TrpcError.Tagged`) with typed `data`
`create({ transformer: superjson })` and each link's `transformer`	`serializer` on the handler and every link; plain JSON by default, danSON with `createSerializer()` (Dates, Maps, BigInts, files)
`fetchRequestHandler({ endpoint, req, router, createContext })`	`createFetchHandler({ endpoint, router, createContext }).fetch(req)`
`createContext({ req, resHeaders })`	`createContext({ request, info })`; headers go on the procedure's `response` handle
adapters (standalone, express, next, aws-lambda, ws)	one handler: see Adapters
`createTRPCClient<AppRouter>({ links })`	`createTrpcClient({ router: routerType<AppRouter>(), links })`
`httpBatchLink`, `httpBatchStreamLink`, `httpSubscriptionLink`	`httpLink({ url, batch: true })` streams and subscribes too
`createWSClient` + `wsLink({ client })`	`wsLink({ url })`
`TRPCClientError`	`TrpcError`; `safe()` for `[data, error]`
`t.createCallerFactory(router)(ctx)`	`createRouterClient(router, { ctx })`
subscriptions returning `observable()`	an async generator or an Effect `Stream`
`input.lastEventId` in a subscription	the resolver's `lastEventId` option
`.subscribe(input, { onData, onError })`	`for await (const event of client.x.subscribe(input))`
`@trpc/react-query`, `@trpc/tanstack-react-query`	`@trpc12/tanstack-react-query`
`abortOnUnmount: true`	always on: queries pass TanStack's `signal`, so one nothing observes is cancelled