Add support for Operator '+'

[kenjones-cisco]

Add support for Operator '+'

The operator + takes two filters, applies them both to the same input, and adds the results together. What “adding” means depends on the types involved:

Numbers are added by normal arithmetic.

Arrays are added by being concatenated into a larger array.

Strings are added by being joined into a larger string.

Objects are added by merging, that is, inserting all the key-value pairs from both objects into a single combined object. If both objects contain a value for the same key, the object on the right of the + wins. (For recursive merge use the * operator.)

null can be added to any value, and returns the other value unchanged.