feat(solutions): add sorting-algorithms/javascript/merge-sort

challenges/sorting-algorithms/solutions/javascript/merge-sort/README.md

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+# sorting-algorithms/javascript/merge-sort
+
+Created by [@Divlo](https://github.com/Divlo) on 29 June 2021.
+
+| Algorithm                                                   | Best Case   | Average Case | Worst Case  |
+| ----------------------------------------------------------- | ----------- | ------------ | ----------- |
+| [Merge sort](https://wikipedia.org/wiki/Merge_sort)         | O(n log(n)) | O(n log(n))  | O(n log(n)) |

challenges/sorting-algorithms/solutions/javascript/merge-sort/package.json

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+{
+  "type": "module"
+}

challenges/sorting-algorithms/solutions/javascript/merge-sort/solution.js

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+import readline from 'readline'
+
+const input = []
+const readlineInterface = readline.createInterface({
+  input: process.stdin,
+  output: process.stdout
+})
+readlineInterface.on('line', (value) => {
+  input.push(value)
+})
+readlineInterface.on('close', solution)
+
+function solution() {
+  const numbers = input.map((value) => Number(value))
+  const sortedNumbers = mergeSort(numbers)
+  sortedNumbers.forEach((number) => {
+    console.log(number)
+  })
+}
+
+function divideArray (numbers) {
+  const middle = Math.round(numbers.length / 2)
+  const left = numbers.slice(0, middle)
+  const right = numbers.slice(middle)
+  return [left, right]
+}
+
+function merge (numbers1, numbers2) {
+  let indexNumbers1 = 0
+  let indexNumbers2 = 0
+  const result = []
+  while (indexNumbers1 < numbers1.length && indexNumbers2 < numbers2.length) {
+    if (numbers1[indexNumbers1] < numbers2[indexNumbers2]) {
+      result.push(numbers1[indexNumbers1])
+      indexNumbers1 += 1
+    } else {
+      result.push(numbers2[indexNumbers2])
+      indexNumbers2 += 1
+    }
+  }
+  for (let index = indexNumbers1; index < numbers1.length; index++) {
+    result.push(numbers1[index])
+  }
+  for (let index = indexNumbers2; index < numbers2.length; index++) {
+    result.push(numbers2[index])
+  }
+  return result
+}
+
+function mergeSort (numbers) {
+  if (numbers.length <= 1) {
+    return numbers
+  }
+  let [left, right] = divideArray(numbers)
+  left = mergeSort(left)
+  right = mergeSort(right)
+  return merge(left, right)
+}