Editorial: normalize property name iteration

[jugglinmike]

Prior to this commit, the variable named prop was not defined in two
of the loops within ToDateTimeOptions. Define it using the convention
for loops in ECMA262, and update the algorithm's similar loops to match.

[ryzokuken]

Looks good overall. I'm wondering here that since there's nothing quite special about being a "property name" and since we're just iterating a constant list, it should instead be For each element _prop_ of « ... » to simply things.

[jugglinmike]

The wording in this patch is based on a similar step in the BasicFormatMatcher algorithm. There, the qualifier is necessary to disambiguate between other columns in the referenced table. That's not a concern here, but it could still be useful as a way to further contextualize the purpose of the list elements. That said, I'm happy to change this according to the editors' preference.

[ryzokuken]

@jugglinmike you're right. Besides, either way, this is a net positive with respect to the status quo so let's merge this and discuss any further changes separately.

Thanks for the PR again! 😇