Move internal modules to “internal” folder.

.internal/setToString.js

@@ -0,0 +1,14 @@
+import baseSetToString from './.internal/baseSetToString.js';
+import shortOut from './.internal/shortOut.js';
+
+/**
+ * Sets the `toString` method of `func` to return `string`.
+ *
+ * @private
+ * @param {Function} func The function to modify.
+ * @param {Function} string The `toString` result.
+ * @returns {Function} Returns `func`.
+ */
+const setToString = shortOut(baseSetToString);
+
+export default setToString;