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.

default.nix

@@ -0,0 +1,14 @@
+(
+  import
+  (
+    let
+      lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+    in
+      fetchTarball {
+        url = lock.nodes.flake-compat.locked.url or "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+        sha256 = lock.nodes.flake-compat.locked.narHash;
+      }
+  )
+  {src = ./.;}
+)
+.defaultNix