feat: move self-defined packages to own files

pkgs/cdb/default.nix

@@ -0,0 +1,9 @@
+{ writeShellApplication, fzf }:
+
+writeShellApplication {
+  name = "cdb";
+  runtimeInputs = [ fzf ];
+  text = ''
+    git checkout "$(git branch --list | grep -v "^\*" | fzf | awk '{print $1}')"
+  '';
+}