Add array utility functions

[dev-dami]

Description

Need more array manipulation functions in the standard library.

Suggested Functions

• map(arr, fn) - Transform each element
• filter(arr, fn) - Filter elements by predicate
• reduce(arr, fn, initial) - Reduce to single value
• find(arr, fn) - Find first matching element
• contains(arr, element) - Check if element exists
• reverse(arr) - Reverse array
• sort(arr) - Sort array

Where to Start

• pkg/eval/builtins.go - Add builtin function implementations
• May need to handle function arguments (closures)

Good First Issue

Great for newcomers! Start with simpler ones like contains, reverse, then work up to map/filter.