Add extracted source directory and README navigation

extracted-source/node_modules/lodash-es/_baseSetToString.js

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+import constant from './constant.js';
+import defineProperty from './_defineProperty.js';
+import identity from './identity.js';
+
+/**
+ * The base implementation of `setToString` without support for hot loop shorting.
+ *
+ * @private
+ * @param {Function} func The function to modify.
+ * @param {Function} string The `toString` result.
+ * @returns {Function} Returns `func`.
+ */
+var baseSetToString = !defineProperty ? identity : function(func, string) {
+  return defineProperty(func, 'toString', {
+    'configurable': true,
+    'enumerable': false,
+    'value': constant(string),
+    'writable': true
+  });
+};
+
+export default baseSetToString;