Added Dart examples for chapter 3 to chapter 9 (#265)

* fix: corrected method return value following Dart's newest linter version

* feat: added Dart recursion examples for chapter 3

* feat: added quicksort example in Dart for chapter 4

* feat: added examples in Dart for the chapter 5

* feat: added Dart example for chapter 6 bfs

* feat: added djikstra example in Dart for chapter 7

* feat: added example of set covering in Dart for chapter 8

* feat: added examples for dynamic programming in dart for chapter 9

03_recursion/dart/01_countdown/countdown.dart

@@ -0,0 +1,14 @@
+main() {
+  final Stopwatch stopwatch = Stopwatch()..start();
+  print(recursiveCount([0, 21, 3, 1, 6, 5, 81, 2, 14, 56, 32, 1, 9, 8]));
+  stopwatch.stop();
+  print(stopwatch.elapsedMilliseconds);
+}
+
+int recursiveCount(List array) {
+  if (array.isEmpty) {
+    return 0;
+  }
+  final List newArray = [...array]..removeAt(0);
+  return 1 + recursiveCount(newArray);
+}

03_recursion/dart/02_sum/sum.dart

@@ -0,0 +1,14 @@
+void main(List<String> args) {
+  final Stopwatch stopwatch = Stopwatch()..start();
+  print(recursiveSum([0, 21, 3, 1, 6, 5, 81, 2, 14, 56, 32, 1, 9, 8]));
+  stopwatch.stop();
+  print(stopwatch.elapsedMilliseconds);
+}
+
+int recursiveSum(List<int> array) {
+  if (array.isEmpty) {
+    return 0;
+  }
+  final List<int> newArray = [...array]..removeAt(0);
+  return array[0] + recursiveSum(newArray);
+}

03_recursion/dart/03_factorial/factorial.dart

@@ -0,0 +1,13 @@
+void main(List<String> args) {
+  final Stopwatch stopwatch = Stopwatch()..start();
+  print(recursiveFactorial(5));
+  stopwatch.stop();
+  print(stopwatch.elapsedMilliseconds);
+}
+
+int recursiveFactorial(int value) {
+  if (value == 1) {
+    return 1;
+  }
+  return value * recursiveFactorial(value - 1);
+}