feat(javascript): add a flatMap, array method

languages/javascript.js

@@ -94,3 +94,4 @@ arr.reduce(callback[, initialValue])                 // Apply a function against
 arr.reduceRight(callback[, initialValue])            // Apply a function against an accumulator and each value of the array (from right-to-left) as to reduce it to a single value.
 arr.some(callback[, initialValue])                   // Returns true if at least one element in this array satisfies the provided testing function.
 arr.values()                                         // Returns a new Array Iterator object that contains the values for each index in the array.
+arr.flatMap(callback[, thisArg])                     // Apply the callback function for each element in array and flatten the result in a new array