Add undefined to return type of array-last

[sunnyzanchi]

It's possible for array-last to return undefined, so it should be included in the return type. If there's a more sophisticated typing that makes sense here i'm happy to change it

✔️ Updated tests and yarn test-types passes

[angus-c]

Not a TS expert so I may be wrong, but I don't this change is necessary.
See https://tinyurl.com/2p9fjbct. (this is with StrictNullChecks on)

Also in index.tests.ts probably just needs
const test8: undefined = last([undefined]);

[sunnyzanchi]

in this example, the definition of last errors with dynamic arrays — i think this is maybe from a newer version of TS, i don't remember this happening before. last2 doesn't let TS catch that d might be undefined. i think the type definition of last3 is the only one that covers all cases