reproducible dev environment via flakes

[harryprayiv]

• adding flakes to allow us nix people to work on this without installing ruby into our main environment
• updated json dependency but my flake isn't connected to your gemfile at the moment because

I still get the same deprecated warning from Ruby that looks like this on my system:

/nix/store/j1c8qr0ry0ilqz069m76zqkwk2cnvjl8-ruby2.7.6-json-2.2.0/lib/ruby/gems/2.7.0/gems/json-2.2.0/lib/json/common.rb:156: warning: Using the last argument as keyword parameters is deprecated
/nix/store/j1c8qr0ry0ilqz069m76zqkwk2cnvjl8-ruby2.7.6-json-2.2.0/lib/ruby/gems/2.7.0/gems/json-2.2.0/lib/json/common.rb:156: warning: Using the last argument as keyword parameters is deprecated