feat: add unaryReduceSubarrayBy to namespace

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---

Author: kgryte

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

@@ -1066,6 +1066,15 @@ setReadOnly( ns, 'unaryReduceStrided1dDispatchFactory', require( '@stdlib/ndarra
 */
 setReadOnly( ns, 'unaryReduceSubarray', require( '@stdlib/ndarray/base/unary-reduce-subarray' ) );
 
+/**
+* @name unaryReduceSubarrayBy
+* @memberof ns
+* @readonly
+* @type {Function}
+* @see {@link module:@stdlib/ndarray/base/unary-reduce-subarray-by}
+*/
+setReadOnly( ns, 'unaryReduceSubarrayBy', require( '@stdlib/ndarray/base/unary-reduce-subarray-by' ) );
+
 /**
 * @name unaryStrided1d
 * @memberof ns