Switch @bufbuild/connect-web to use @bufbuild/connect

[timostamm]

This updates @bufbuild/connect-web to share logic with @bufbuild/connect-node. Shared types like ConnectError, createPromiseClient, and others live in @bufbuild/connect, and are re-exported from the other packages.

This switch comes with the following improvements and bug fixes:

• Allow JSON in the gRPC-web transport of @bufbuild/connect-web #334
• Emit unpadded base64 for binary headers #454
• Add a cause property to ConnectError #407

This also changes the interfaces Interceptor and Transport.

To understand this change, it is best to compare the old Transport with the new Transport. Transport now has a method stream() instead of just serverStream(), and this method takes an AsyncIterable<I> as input. This allows us to implement full duplex streaming via the Node.js http2 package.

This means that the types related to interceptors also had to change (old vs new), but note that a simple interceptor that adds a request header still looks the same:

function addHeaderInterceptor(value: string): Interceptor {
  return (next) => async (req) => {
    req.header.set("foo", value);
    return await next(req);
  };
}

And intercepting streams actually becomes more straight-forward, thanks to asynchronous iterables:

function streamLoggingInterceptor(): Interceptor {
  async function* logStream(stream: AsyncIterable<Message>) {
    for await (const m of stream) {
      console.log("saw streaming response message", m);
      yield m;
    }
  }
  return (next) => async (req) => {
    const res = await next(req);
    if (res.stream) {
      return {...res, message: logStream(res.message)};
    }
    console.log("saw unary response message", res.message);
    return res;
  };
}