add project to nix packages

[zoranbosnjak]

Several sanic-* packages are already included in nix packages, but not sanic-ext. It would be very convenient if this package is also included.

I have experimented with nix build myself and I had to remove the setuptools<60.0 version restriction to build. Not sure if this restriction is required for some other reasons.
See main...zoranbosnjak:sanic-ext:main.