Add support for Next.js

[timostamm]

This adds basic support for Next.js via the new package @bufbuild/connect-next. It passed our tests, but still needs to be added to our test suite.

The setup includes creating two files:

1. connect.ts is where you register your RPCs:

// connect.ts
import { ConnectRouter } from "@bufbuild/connect";

export default function(router: ConnectRouter) {
  // implement rpc Say(SayRequest) returns (SayResponse)
  router.rpc(ElizaService, ElizaService.methods.say, async (req) => ({
    sentence: `you said: ${req.sentence}`,
  }));
}

2. pages/api/[[..connect]].ts is a Next.js catch-all API route:

// pages/api/[[..connect]].ts
import { nextJsApiRouter } from "@bufbuild/connect-next";
import routes from "../../connect";

const {handler, config} = nextJsApiRouter({ routes });
export {handler as default, config};

With that server running, you can make requests with any Connect or gRPC-Web client.
Note that Next.js serves all your RPCs with the /api prefix.

Currently, @bufbuild/connect-next does not support the Vercel Edge runtime. It requires the Node.js server runtime, which is used by default when deploying to Vercel.