Add "builtAt" to builders

ListBuilder could for example have a "builtAt" method that converts the value at the specified index to a builder, similar to the "rebuild" method but for an element in the collection:

var list = new BuiltList>([new BuiltList[1]]);
var rebuiltList = list.rebuild((b) => b.rebuildAt(0).add(2));

--> [[1, 2]]

This should work for nested built collections and for built values.