Rename @bufbuild/connect-core to @bufbuild/connect

[timostamm]

We are changing our approach with @bufbuild/connect-core after running some spikes with support for various web frameworks.

We are moving towards @bufbuild/connect being the single entry point for users, with platform- or framework-specific adapters imported from - for example - @bufbuild/connect-node.

This adds a new primitive to Connect for ES: ConnectRouter. It serves as a convenient and central place to register services or individual RPC implementations:

// 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}`,
  }));
}