Array(Push): Add push definition and exercise

arrays/push.md

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+#push
+
+The push method add one or more elements to the end of the original array and returns the new length of the array.
+It mutates the original array.
+
+```javascript
+let arr = [45, 96, 73, 58, 23]
+
+// arr: [45, 96, 73, 58, 23, 10]
+// Result: len = 6 
+let len = arr.push(10)
+```
+
+{% exercise %}
+Use the push method to insert 23 at the end of the array RMS and calculate new length of RMS.
+{% initial %}
+let RMS = [ 100, 20, 55, 87, 91]
+let len =
+{% solution %}
+let RMS = [ 100, 20, 55, 87, 91]
+let len = RMS.push(23);
+{% validation %}
+assert (len === 6)
+{% endexercise %}
+