Make firstDayOfWeek return string and fix spec bug

[FrankYFTang]

This revert our change after TC39 Sept meeting so

I. Intl.Locale.prototype.firstDayOfWeek returns any strings which fit type = alphanum{3,8} (sep alphanum{3,8})*. as defined in UTS35
2. {firstDayOfWeek} for new Intl.Locale() accepts: strings fit Default: undefined

[FrankYFTang]

Sorry. I just realize this PR is not good enough. Need more work. I need to rework on this. Intl.Locale.prototype.firstDayOfWeek need to return string, but not just that 7 string values.

[FrankYFTang]

• Consensus reach to merge PR 79 to align other Intl.Locale API in TC39 2023-11-28 meeting

[FrankYFTang]

@sffc @anba @ben-allen ping

[trflynn89]

Hey @FrankYFTang - While implementing this, I came across a small issue. I think WeekInfoOfLocale needs to be updated to match the [[FirstDayOfWeek]] changes. [[FirstDayOfWeek]] can now be an arbitrary string, but steps 4-5 of WeekInfoOfLocale state:

4. Let fw be loc.[[FirstDayOfWeek]].
5. If fw is not undefined, then
    a. Set r.[[FirstDay]] to fw.

Where r.[[FirstDay]] is an "Integer value between 1 and 7". So I think there needs to be a step to convert fw from a string to an integer here.

[sosukesuzuki]

@FrankYFTang @ptomato Maybe we should update test262 for this?

[ptomato]

It's not clear to me what changed just from looking at the PR, but yes please do update test262 if it's needed.

[FrankYFTang]

Hey @FrankYFTang - While implementing this, I came across a small issue. I think WeekInfoOfLocale needs to be updated to match the [[FirstDayOfWeek]] changes. [[FirstDayOfWeek]] can now be an arbitrary string, but steps 4-5 of WeekInfoOfLocale state:

4. Let fw be loc.[[FirstDayOfWeek]].
5. If fw is not undefined, then
    a. Set r.[[FirstDay]] to fw.

Where r.[[FirstDay]] is an "Integer value between 1 and 7". So I think there needs to be a step to convert fw from a string to an integer here.

fork that issue into #82