Add first-class flake-edit follow support

[dnlzro]

Summary

Add an opt-in auto-follow feature powered by flake-edit follow:

{
  flake-file.auto-follow.enable = true;
}

When enabled, write-flake updates the lock file, runs flake-edit follow, formats the resulting flake.nix, and updates the lock file a second time. I don't love that the lockfile has to be generated twice, but...

• The first lock creates the lockfile that flake-edit follow needs in order to determine which follows entries to produce.
• The second lock reconciles with the changes that flake-edit follow made to flake.nix.

---

check-flake-file now performs the same operation on a temporary copy (to detect missing or stale automatic follows). I needed to make changes so that the check did not treat flake-edit's changes as an unexplained diff.

Note

This was my primary motivation for making this PR. Although it is possible to wire up flake-edit follow using flake-file.write-hooks and get the expected output, check-flake-file doesn't account for write-hooks that that re-write flake.nix itself, so the check fails.

---

Automatic following can be disabled for a particular nested input:

{
  flake-file.inputs.crane.inputs.nixpkgs.autoFollow = false;
}

An input cannot have both a follows entry and autoFollow = false configured, since those settings request contradictory behavior.

Auto-follow requires pkgs.flake-edit 0.3.5 or newer, and reports an error when that requirement is not met.

---

The code is tested, but maybe not as extensively as you'd like. I'm happy to improve on that if necessary.

I also added some documentation for this new feature, and included trade-offs between it and the lock-flattening approach already supported by flake-file. I will refine the docs a bit if you're interested in getting this merged.

Closes #131.