Add core List combinators to the standard library

[MohibShaikh]

What

Adds 12 core List combinators to the standard library (src/fun/builtins.bend):

List/map, List/fold, List/sum, List/all, List/any, List/find, List/contains, List/take, List/drop, List/at, List/zip, List/range.

Why

The List module currently only exposes length, reverse, split_once, flatten, concat, and filter, so the everyday functional toolkit (map/fold/take/zip/…) has to be re-implemented by every user. These are the most commonly reached-for list operations.

Notes

• All functions follow the existing idiom: fold for catamorphic reductions (map, fold, sum) and structural match-recursion where early-exit or index tracking is needed (all, any, find, take, drop, at, zip). No imperative loops.
• List/sum and List/contains are u24-specialized because arithmetic/equality operators only apply to native numbers and there is no generic Eq/Num interface in the stdlib; the rest are generic over the element type.
• List/find and List/at return Maybe(T); List/at returns Maybe/None for out-of-bounds indices.
• List/take/List/drop clamp gracefully when n exceeds the length; List/zip truncates to the shorter list; List/range is half-open [start, end).

Tests & docs

• Added a golden test tests/golden_tests/run_file/list_combinators.bend exercising all 12 functions (including the clamp/truncation edge cases), with a recorded snapshot.
• Documented each function in docs/builtins.md and added a CHANGELOG entry.