fix(types): nested object can be a oneof

[alexander-fenster]

Fixing the type declaration to allow one very special case: a field extension that is a proto3 optional field. Internally, proto3 optional fields are represented as a single field oneofs, which makes it the first case ever when a oneof can be a nested object.

Example: bigquery-storage

extend google.protobuf.FieldOptions {
  optional string column_name = 454943157;
}

Related part of the JSON:

                      "nested": {
                        "_columnName": {
                          "oneof": [       <--- this is not accepted without the fix
                            "columnName"
                          ]
                        },
                        "columnName": {
                          "type": "string",
                          "id": 454943157,
                          "extend": "google.protobuf.FieldOptions",
                          "options": {
                            "proto3_optional": true
                          }
                        },

Yeah, also, last but not least, VSCode does not hang anymore, so removing that comment. It was added 6 years ago, not true anymore.