Add mutable registry

[timostamm]

This PR adds a mutable registry:

import { createMutableRegistry } from "@bufbuild/protobuf";
import { MyMessage } from "./gen/example_pb";

const registry = createMutableRegistry();

// Adds a message, enum, extension, or service type to the registry
registry.add(MyMessage);

Closes #679.

Similar functionality is added to v2 in #898.