Add Nix/NixOS dev-shell (#13407)

This PR adds a Nix/NixOS development-shell (`shell.nix`), which is based
on the upstream
[nixpkgs](c5d4d45811/pkgs/by-name/ze/zed-editor/package.nix),
as well as its corresponding `flake.nix` file.

To use it, run either the `nix-shell` command (uses the `shell.nix`
file), or the newer but experimental `nix develop` command (uses
`flake.nix`)

~~This has not been tested on macOS, tho preliminary code is there to
try and support it, feel free to report any issues.~~ Zed unfortunately
doesn't build on nix-darwin (see
https://github.com/NixOS/nixpkgs/issues/320084), so this PR doesn't aim
to add darwin support.

---

Release Notes:

- N/A

---------

Signed-off-by: xtrm <oss@xtrm.me>
Co-authored-by: Niklas Korz <niklas@niklaskorz.de>

.gitignore

@@ -1,3 +1,4 @@
+/.direnv
 .idea
 **/target
 **/cargo-target