Add Promise.allSettled and AllSettledResultElement

[niloc132]

Fixes #182

[niloc132]

This is a quick sketch of what this likely should look like, based on Promise.all() and race(). Note that the AllSettledResultElement would technically have setters and a create() method, but there is no plausible mechanism for a user to create these elements directly.

Likely there are some specific desired @JsNullable/@Nullable expectations - happy to apply any desired changes.