fix: use children.extend in Menu.add

[wasinski]

Hi, I just started using this library via Litestar's plugin and I ran into an issue that only occurred when I was running tests of my application. When I have used the app normally then I could not get this error.

The error:

>               root.children.append(*item.children)
E               TypeError: list.append() takes exactly one argument (0 given)

It's raised when children are an empty list. Fix was easy, so I hope it's not expected that at that point children aren't empty, but if it was so, then probably a nicer error should be raised.