Fix transition typings

[mattyclarkson]

Allows assigning multiple transition prooperties now. keyof S[]
is the keys of an array of the generic property which is length,
etc.

[mattyclarkson]

@aminroosta does this make sense to you? You added the original typings. I couldn't pass an array of transition strings without his change.

[aminroosta]

@mattyclarkson according to this stackoverflow question (Array<Type> vs Type[]), i think both keyof s[] and Array<keyof s> should be equivalent. Now that the keyof s[] is not working let's use Array<keyof s>.

LGTM 👍

[mattyclarkson]

@aminroosta I think in the general case that is correct. However, I don't know the precedence of the keyof operator in the type system. Is keyof S[] interpreted as *the keys of an array of S (i.e. "length" | "push" | "etc") * or an array of keys of S (i.e. ["color" | "etc"]). I think I was hitting the former, when we need the latter. Array<keyof S> removes the ambiguity. I think (keyof S)[] would do the same thing, maybe. Thanks for the approval 😄