chore: Add nix development flake

[ixhbinphoenix]

Adds a nix flake that supplies a dev shell with rust nightly and node+pnpm. Needed for NixOS users to work on this project efficiently.

Also adds a rust-toolchain.toml to accurately reflect the currently used toolchain

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Name	Status	Preview	Updated (UTC)
bne	✅ Ready (Inspect)	Visit Preview	Aug 20, 2023 1:30am

[ixhbinphoenix]

pls someone review this ASAP so i can not work again