Rename List.split to List.splitAt and add List.splitOn and List.splitOnList

[isaacvando]

This implements #6164.

[lukewilliamboswell]

Seems a little odd that this only failed in nix

---- gen_list::list_split_on_list stdout ----
thread 'gen_list::list_split_on_list' panicked at crates/compiler/mono/src/ir/decision_tree.rs:777:17:
List { arity: Slice(1, 0), list_layout: InLayout(22), element_layout: InLayout(VOID), elements: [Identifier(`List.IdentId(512)`)], opt_rest: Some((1, Some(`List.IdentId(513)`))) }

[isaacvando]

I'm going to add Str.splitOn to this PR also so that both functions can be renamed at the same time.

[Anton-4]

Thanks @isaacvando :)