Provide a strong type name in the generated message schema

[timostamm]

From #994 (comment):

It would be very convenient to include a strongly typed type name in the schema for a generated message. For example:

syntax = "proto3";
package example;
message User {
  string first_name = 1;
}

This definition generates a type User and a const UserSchema. The type's $typeName property is strongly typed, but the schema's typeName property is not:

const user: User = create(UserSchema);
user.$typeName; // "example.User"

UserSchema.typeName; // string

This means the schema's property cannot be used to construct an instance of the type, and it cannot be used to narrow down a message by using the property as a descriminator.

If it's possible without breaking changes, we should change the generated schema to support this.