Add NixOS build instructions for Tabnine Chat

[seanrmurphy]

Is your feature request related to a problem? Please describe.
The repo contains no info on building Tabnine Chat for NixOS - I had to work it out myself.

Describe the solution you'd like
Add something like the following shell.nix to the chat directory to the repo:

# built against 960fae2187687ff0929775ffedb6f05172b990d2 with nixos 24.05

{ pkgs ? import <nixpkgs> {} }:
  pkgs.mkShell rec {
    buildInputs = with pkgs; [
      cargo
      rustc
      pkg-config
    ];

    nativeBuildInputs = with pkgs; [
      glib
      gdk-pixbuf
      libsoup_3
      pango
      gtk3
      webkitgtk_4_1
    ];
  }

and this can be used to start a shell with nix-shell which has the correct configuration to build Tabnine Chat.

Describe alternatives you've considered
None - it prob would be easy to create a flake which would perform the build - I could look into this if it would be useful.

Additional context
None.

[amirbilu]

PRs are welcome :)

[seanrmurphy]

Done! #178