feat: add flattenBy to namespace

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---

Author: kgryte

lib/node_modules/@stdlib/ndarray/lib/index.js

@@ -279,6 +279,15 @@ setReadOnly( ns, 'flags', require( '@stdlib/ndarray/flags' ) );
 */
 setReadOnly( ns, 'flatten', require( '@stdlib/ndarray/flatten' ) );
 
+/**
+* @name flattenBy
+* @memberof ns
+* @readonly
+* @type {Function}
+* @see {@link module:@stdlib/ndarray/flatten-by}
+*/
+setReadOnly( ns, 'flattenBy', require( '@stdlib/ndarray/flatten-by' ) );
+
 /**
 * @name forEach
 * @memberof ns