新增代码文件支持行号显示

pub/code_field-master/example/lib/readme/fib.dart

@@ -0,0 +1,13 @@
+// An expensive but pretty
+// recursive implementation
+int fibonacci(int n) {
+  if (n <= 1) return n;
+  return fibonacci(n - 1)
+    + fibonacci(n - 2);
+}
+
+void main() {
+  print("Fibonacci sequence:");
+  for (var n = 0; n < 10; n++)
+    print(fibonacci(n));
+}