Mathematical sum function

**Is your feature request related to a problem? Please describe.**
Each year I try to solve a few advent-of-code problems with Bicep to see how far one can get. I noticed there is no `sum()` function. I am sure this function can come in handy even in real-world Bicep problems, even if I can't think of a legit use-case right now.

**Describe the solution you'd like**
I would like to have a `sum` function that sums an array of integers. Example:

```bicep
var mySum = sum([1,2,3]) // mySum = 6
```

Of course there is a `reduce` function, but for such a trivial operation a `sum` function would make more sense.

Start by reading the issue's existing reduce function reference and locate where Bicep array functions are defined and tested. The requested behavior is a sum function accepting an array of integers, with sum([1,2,3]) producing 6; completion should include coverage for that example and the relevant edge cases.

Written by the indexing model from the issue text.