wip: feat: Emacs config (from scratch }:3)

Working:
  - The daemon
  - Mutable config — well, kinda. I'm not yet sure how it will interact with
    `emacsWithPackagesFromUsePackage`.
  - Packages via Nixpkgs

flake.nix

@@ -56,4 +56,13 @@
 
       homeConfigurations = import ./outputs/homeConfigurations.nix inputs;
     };
+
+  nixConfig = {
+    extra-substituters = [
+      "https://nix-community.cachix.org"
+    ];
+    extra-trusted-public-keys = [
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+    ];
+  };
 }