feat: rewrite programming-challenges CLI (#3)

challenges/hello-world/solutions/dart/function/README.md

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+# hello-world/dart/function
+
+Created by [@Divlo](https://github.com/Divlo) on 6 June 2021.

challenges/hello-world/solutions/dart/function/solution.dart

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+import 'dart:io';
+
+String readLineSync() {
+  String? string = stdin.readLineSync();
+  return string == null ? '' : string;
+}
+
+void main() {
+  String input = readLineSync();
+  print('Hello, $input!');
+}