This work was done by Jerónimo Milea. Jerónimo let me know he was working on DDEX support on this thread after I said I was working on DDEX support for Npgsql.
He sent me his working copy and I started to play with it. Note that as a preview version many things may not work ok and we wanted you to provide us feedback so we can fix any bugs.
Design time support is provided as a zip file containing support code as well as a copy of Npgsql project file. So, everything you need to start working with design time support is already packaged for you.