Currently, there's no way to append key-value pairs to a list builder object after construction. We should provide an append method for this case.

Defining an operator+= method will allow users to stick to the JSON-like syntax on the expression's right-hand side. For example:

list::builder doc = {"hello", "world"};
doc += {"foo", "bar"};

instead of

list::builder doc = {"hello", "world"};
doc.append({"foo", "bar"});