fix: support for nested messages and enums within group blocks

[JoshuaWise]

This fixes support for proto files that have nested messages or enums declared within groups, like this:

syntax = "proto2";

message Foo {
  repeated group Field = 1 {
    message Bar {
      optional bool my_bool = 2;
    }

    optional string my_string = 3;
    optional Bar my_bar = 4;
  }
}

[JoshuaWise]

Closed in favor of #1790