Implement Array.prototype.findLast and Array.prototype.findLastIndex

[Devin Rousso]

`Array.prototype.findLast` and `Array.prototype.findLastIndex` just advanced to TC39 stage 3

Proposal: <https://github.com/tc39/proposal-array-find-from-last>
Spec: <https://tc39.es/proposal-array-find-from-last/index.html>

```
const array = [{ value: 1 }, { value: 2 }, { value: 3 }, { value: 4 }];

array.find(n => n.value % 2 === 1); // { value: 1 }
array.findIndex(n => n.value % 2 === 1); // 0

array.findLast(n => n.value % 2 === 1); // { value: 3 }
array.findLastIndex(n => n.value % 2 === 1); // 2
```

[Devin Rousso]

Created attachment 433476 [details]
[Patch] WIP

[Yusuke Suzuki]

Comment on attachment 433476 [details]
[Patch] WIP

View in context: https://bugs.webkit.org/attachment.cgi?id=433476&action=review

r=me

> Source/JavaScriptCore/builtins/ArrayPrototype.js:269
> +        var kValue = array[i];

I like the name like `element` instead of `kValue` even though it is derived from the spec's name since the name is not suitable for WebKit's coding style.

[Yusuke Suzuki]

Comment on attachment 433476 [details]
[Patch] WIP

Can you add tests in JSTests/stress?

[Devin Rousso]

Created attachment 433521 [details]
Patch

[Devin Rousso]

Created attachment 433528 [details]
Patch

[Devin Rousso]

Created attachment 433547 [details]
Patch

[EWS]

Committed r279937 (239681@main): <https://commits.webkit.org/239681@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 433547 [details].

[Radar WebKit Bug Importer]

<rdar://problem/80610784>