V2: Add mutable registry

[timostamm]

This PR adds a mutable registry:

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

const registry = createMutableRegistry();

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

// Removes a descriptor
registry.remove(MyMessageSchema);

registry.getMessage("MyMessage"); // undefined

Note that registries - mutable and immutable ones - also support composition:

import { createRegistry } from "@bufbuild/protobuf";
import { MyMessageSchema, file_example } from "./gen/example_pb";

// The variadic function accepts any number of arguments
const registry = createRegistry(
  MyMessageSchema, // Initialize with a message, enum, extension, or service descriptor
  file_example, // Passing a file descriptor adds all types defined in the Protobuf file
  otherRegistry, // Adds all types from the other registry
);

Closes #679.