feat(solutions): add sorting-algorithms/dart/insertion-sort

challenges/sorting-algorithms/solutions/dart/insertion-sort/README.md

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+# sorting-algorithms/dart/insertion-sort
+
+Created by [@Divlo](https://github.com/Divlo) on 29 June 2021.
+
+| Algorithm                                                   | Best Case   | Average Case | Worst Case  |
+| ----------------------------------------------------------- | ----------- | ------------ | ----------- |
+| [Insertion sort](https://wikipedia.org/wiki/Insertion_sort) | O(n)        | O(n²)        | O(n²)       |

challenges/sorting-algorithms/solutions/dart/insertion-sort/solution.dart

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+import 'dart:io';
+
+void main() {
+  int length = int.parse(readLineSync());
+  List<int> numbers = [];
+  for (int indexInput = 0; indexInput < length; indexInput++) {
+    numbers.add(int.parse(readLineSync()));
+  }
+  List<int> sortedNumbers = insertionSort(numbers);
+  for (int index = 0; index < length; index++) {
+    print(sortedNumbers[index]);
+  }
+}
+
+List<int> insertionSort(Iterable<int> numbersInput) {
+  var numbers = [...numbersInput];
+  for (var index1 = 0; index1 < numbers.length; index1++) {
+    var current = numbers[index1];
+    var index2 = index1 - 1;
+    while (index2 >= 0 && numbers[index2] > current) {
+      numbers[index2 + 1] = numbers[index2];
+      index2 -= 1;
+    }
+    numbers[index2 + 1] = current;
+  }
+  return numbers;
+}
+
+String readLineSync() {
+  String? string = stdin.readLineSync();
+  return string == null ? '' : string;
+}