add default snowflake lib and shell as example

lib/default.nix

@@ -0,0 +1,18 @@
+{
+# This is the merged library containing your namespaced library as well as all libraries from
+# your flake's inputs.
+lib,
+
+# Your flake inputs are also available.
+inputs,
+
+# Additionally, Snowfall Lib's own inputs are passed. You probably don't need to use this!
+snowfall-inputs, }: {
+  # This will be available as `lib.my-namespace.my-helper-function`.
+  my-helper-function = x: x;
+
+  my-scope = {
+    # This will be available as `lib.my-namespace.my-scope.my-scoped-helper-function`.
+    my-scoped-helper-function = x: x;
+  };
+}