feat(fish): add shell shortcut registry

registry/shell-shortcut.nix

@@ -0,0 +1,21 @@
+{ lib }:
+let
+  entryType = lib.types.submodule {
+    options = {
+      command = lib.mkOption {
+        type = lib.types.str;
+        example = "nvim";
+        description = "Command expanded by this shell shortcut.";
+      };
+
+      abbreviation = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        description = "Whether fish realizes this shell shortcut as an abbreviation instead of an alias.";
+      };
+    };
+  };
+in
+{
+  type = lib.types.coercedTo lib.types.str (command: { inherit command; }) entryType;
+}