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.
