docs: add options for `protoc-gen-env` and `protoc-gen-default` to registry (#18507)

Reserved field options used for `protoc-gen-env` and `protoc-gen-default`, implemented here: https://github.com/MarnixBouhuis/confpb

I've recently released a protoc plugin for generating defaults / binding environment variables to go protobuf structs. For this I would like to reserve the field options used.

Closes #18507

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/18507 from MarnixBouhuis:patch-1 63e3440062
PiperOrigin-RevId: 684103559

docs/options.md

@@ -521,3 +521,8 @@ with info about your project (name and website) so we can add an entry for you.
 
     *   Website: https://userver.tech/
     *   Extensions: 1240-1249
+
+1.  protoc-gen-default / protoc-gen-env (confpb)
+
+    *   Website: https://github.com/MarnixBouhuis/confpb
+    *   Extensions: 1250-1251