Add extracted source directory and README navigation

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

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+import Symbol from './_Symbol.js';
+import isArguments from './isArguments.js';
+import isArray from './isArray.js';
+
+/** Built-in value references. */
+var spreadableSymbol = Symbol ? Symbol.isConcatSpreadable : undefined;
+
+/**
+ * Checks if `value` is a flattenable `arguments` object or array.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is flattenable, else `false`.
+ */
+function isFlattenable(value) {
+  return isArray(value) || isArguments(value) ||
+    !!(spreadableSymbol && value && value[spreadableSymbol]);
+}
+
+export default isFlattenable;