Add keys to array feature

[Elchi3]

Adding javascript.builtins.Array.@@unscopables and javascript.builtins.Array.@@species to array.

So far the array feature was sort of "pure" and only contained original keys with the same version, but I really don't know where else to sort these two keys. I think if we were to author the array features today, more keys would probably move into the main array feature as they are old enough.

It is unfortunate that web-features doesn't define when such a move into the main feature should happen. I think the project would benefit from having a lintable rule here to avoid being inconsistent. Something like: "If a sub feature is baseline high for more than 3 years, the sub feature should be merged with the main feature." For array, this would mean that features like array-iterators, array-isarray, array-fill, and more would be merged into array, for example. cc @ddbeck

[ddbeck]

Symbol.unscopables is a really weird one. I kinda think it belongs with the with statement. The Symbol itself is not deprecated, but it's part of a broader feature that you shouldn't use.

Symbol.species is similar. It looks like it might go away entirely but even if it doesn't it, I think we could advocate for it to be deprecated in the same sense that the with statement is.

In the mean time, we could park these features here with a TODO and an issue saying that we should reevaluate when have sorted out the discouraged status.

[Elchi3]

Thanks Daniel. I'm closing this PR for now. It sounds like we might need to move all the @@species and @@unscopable symbols from existing features to a new feature on its own (and mark that feature as discouraged once we can).