This adds a Nix flake, shims for 'legacy' Nix, and a `.envrc` for
`direnv`. blisp is now able to run directly via:
`nix run github:pine64/blisp`
and for Nix/NixOS users, this helps with a one-click developer
environment.
Signed-off-by: Dom Rodriguez <shymega@shymega.org.uk>
