add nix flake for dev env #139
                
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Creates a nix flake to manage dev dependencies, same as present in the other related dependency repos, such as web [0]. My goal in submitting this PR is simply to make life easier for developers when switching between repos, provided they've some familiarity with nix.
The nix flake provides a devshell, but does not declare any build targets. There's a
just buildwrapper for convenience.[0] https://github.com/penumbra-zone/web