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

05_hash_tables/dart/01_price_of_groceries.dart

@@ -0,0 +1,15 @@
+void main(List<String> args) {
+  final book = <String, double>{};
+  book.addAll(
+    {
+      'apple': 0.67,
+      'milk': 1.49,
+      'avocado': 1.49,
+    },
+  );
+
+  print(book);
+  print(book['apple']);
+  print(book['milk']);
+  print(book['avocado']);
+}

05_hash_tables/dart/02_check_voter.dart

@@ -0,0 +1,16 @@
+void main(List<String> args) {
+  final voted = <String, bool>{};
+
+  checkVoter('tom', voted);
+  checkVoter('mike', voted);
+  checkVoter('mike', voted);
+}
+
+void checkVoter(String name, Map<String, bool> voted) {
+  if (voted[name] != null) {
+    print('Kick them out!');
+  } else {
+    voted[name] = true;
+    print('Let them vote');
+  }
+}