feat: add Dart benchmarks for binary_trees, delta_blue, fib, for, and method_call

Add five new Dart benchmark implementations (binary_trees, delta_blue, fib, for, method_call) under test/benchmark/ and register the Dart runner using the fletch runtime in util/benchmark.py. The benchmarks are ported from existing Wren and JavaScript versions, covering tree traversal, constraint solving, recursive Fibonacci, loop performance, and method call overhead.

test/benchmark/for.dart

@@ -0,0 +1,13 @@
+main() {
+  var list = [];
+
+  Stopwatch watch = new Stopwatch();
+  watch.start();
+  for (var i = 0; i < 1000000; i++) list.add(i);
+
+  var sum = 0;
+  for (i in list) sum += i;
+
+  print(sum);
+  print("elapsed: ${watch.elapsedMilliseconds / 1000}");
+}