Extracts a contiguous section of a list given a start index and optional length.
The start index is zero-based. Negative indices count from the end of the list. When length is omitted, returns everything from the start index to the end.
Examples
slice(items, 1) → elements from index 1 to end
slice(items, 1, 3) → 3 elements starting at index 1
slice(items, -2) → last 2 elements
slice(items, -3, 2) → 2 elements starting 3 from end