Add Decode and Encode instances for foreign

[the-dr-lazy]

Summary:

• Describe development shell environment in Nix flakes. (This doesn't affect anyone except those use Nix)
• Replace bower with spago. (Sorry for doing that without permission, but I didn't know how to integrate my lsp with bower!)
• Adding Foreign.Decode and Foreign.Encode instances for Refined.

[danieljharvey]

Hello! This all seems fine - the Foreign stuff is great, and adding spago / nix is good, just don't delete the bower.json yet as CI uses it and I think bower is still needed for releasing and pushing to Pursuit etc. If you can put that back then I'm happy to merge this. Thanks for contributing, apologies for the absolutely terrible job I am doing of maintaining!