Add children node to CDP Node representation

Add Node writer. Different CDP messages want different child depths. For now,
only support immediate children, but the new writer should make it easy to
support variable.

src/cdp/testing.zig

@@ -285,6 +285,7 @@ const TestContext = struct {
             _ = self.client.?.serialized.orderedRemove(i);
             return;
         }
+
         std.debug.print("Error not found. Expecting:\n{s}\n\nGot:\n", .{serialized});
         for (self.client.?.serialized.items, 0..) |sent, i| {
             std.debug.print("#{d}\n{s}\n\n", .{ i, sent });