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.

nix/shell.nix

@@ -0,0 +1,52 @@
+{pkgs ? import <nixpkgs> {}}: let
+  stdenv = pkgs.stdenvAdapters.useMoldLinker pkgs.llvmPackages_18.stdenv;
+in
+  if pkgs.stdenv.isDarwin
+  then
+    # See https://github.com/NixOS/nixpkgs/issues/320084
+    throw "zed: nix dev-shell isn't supported on darwin yet."
+  else let
+    buildInputs = with pkgs; [
+      curl
+      fontconfig
+      freetype
+      libgit2
+      openssl
+      sqlite
+      zlib
+      zstd
+      alsa-lib
+      libxkbcommon
+      wayland
+      xorg.libxcb
+      vulkan-loader
+    ];
+  in
+    pkgs.mkShell.override {inherit stdenv;} {
+      nativeBuildInputs = with pkgs; [
+        clang
+        curl
+        perl
+        pkg-config
+        protobuf
+        rustPlatform.bindgenHook
+      ];
+
+      inherit buildInputs;
+
+      shellHook = ''
+        export LD_LIBRARY_PATH="${pkgs.lib.makeLibraryPath ([
+            pkgs.vulkan-loader
+          ]
+          ++ buildInputs)}:$LD_LIBRARY_PATH"
+        export PROTOC="${pkgs.protobuf}/bin/protoc"
+      '';
+
+      FONTCONFIG_FILE = pkgs.makeFontsConf {
+        fontDirectories = [
+          "./assets/fonts/zed-mono"
+          "./assets/fonts/zed-sans"
+        ];
+      };
+      ZSTD_SYS_USE_PKG_CONFIG = true;
+    }