Add sum expression

[TomasEng]

Our expression language needs a sum expression that returns the sum of a list of numbers.

Accptance criteria

• The expression language has a sum function that takes a list of numbers as its only argument
• The function returns the sum of the numbers in the provided list using the same decimal-based algorithm as plus
• The function treats null values as 0
• The function returns 0 when the list is empty