Add zed-editor package and overlay to flake (#16783)

Adds a `zed-editor` package to the flake, along with exported overlay.
Uses [`crane`](https://crane.dev) to avoid issues with updating
git-sourced dependencies' hashes. Crane will also be useful if we want
to export separate packages for `stable`, `preview`, and `nightly` in
the future.

Release Notes:

- Added a default package + overlay to Zed's Nix flake. This is useful
for users wanting to pilot nightly builds of Zed on NixOS.

flake.nix

@@ -3,29 +3,53 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    crane = {
+      url = "github:ipetkov/crane";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
   };
 
-  outputs =
-    { self, nixpkgs }:
-    let
-      inherit (self) outputs;
-      systems = [
-        "aarch64-linux"
+  outputs = {
+    nixpkgs,
+    crane,
+    fenix,
+    ...
+  }: let
+    forAllSystems = function:
+      nixpkgs.lib.genAttrs [
         "x86_64-linux"
-        "aarch64-darwin"
-        "x86_64-darwin"
-      ];
-      forAllSystems = nixpkgs.lib.genAttrs systems;
-    in
-    {
-      devShells = forAllSystems (
-        system:
-        let
-          pkgs = import nixpkgs { inherit system; };
-        in
-        {
-          default = import ./shell.nix { inherit pkgs; };
-        }
-      );
+        "aarch64-linux"
+      ] (system:
+        function (import nixpkgs {
+          inherit system;
+          overlays = [fenix.overlays.default];
+        }));
+  in {
+    packages = forAllSystems (pkgs: let
+      craneLib = (crane.mkLib pkgs).overrideToolchain (p: p.fenix.stable.toolchain);
+      nightlyBuild = pkgs.callPackage ./nix/build.nix {
+        inherit craneLib;
+      };
+    in {
+      zed-editor = nightlyBuild;
+      default = nightlyBuild;
+    });
+
+    devShells = forAllSystems (pkgs: {
+      default = import ./nix/shell.nix {inherit pkgs;};
+    });
+
+    formatter = forAllSystems (pkgs: pkgs.alejandra);
+
+    overlays.default = final: _prev: {
+      zed-editor = final.callPackage ./nix/build.nix {
+        craneLib = (crane.mkLib final).overrideToolchain (p: p.fenix.stable.toolchain);
+      };
     };
+  };
 }