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/fib.dart

@@ -0,0 +1,13 @@
+fib(n) {
+  if (n < 2) return n;
+  return fib(n - 1) + fib(n - 2);
+}
+
+main() {
+  Stopwatch watch = new Stopwatch();
+  watch.start();
+  for (var i = 0; i < 5; i++) {
+    print(fib(28));
+  }
+  print("elapsed: ${watch.elapsedMilliseconds / 1000}");
+}