Reverse the argument order of List.insert

The previous order, insert(element, index), was counter-intuitive.
I'm not aware of any list API that uses this order. I've checked:

* Ruby Array.insert(index, obj...)
* JavaScript array.splice(start, deleteCount[, item1[, item2[, ...]]])
* C++ / QList::insert(int i, const T & value)
* C++ / std::vector::insert
* Lua table.insert (list, [pos,] value)
* C# List<T>.Insert(int index, T item)
* Java Interface List<E>.add(int index, E element)
* Python list.insert(i, x)

So it seemed to me more like an oversight in Wren.

doc/site/core/list.markdown

@@ -17,7 +17,7 @@ Removes all items from the list.
 
 The number of items in the list.
 
-### **insert**(item, index)
+### **insert**(index, item)
 
 **TODO**