Calendar option for non-gregorian calendars

[rxaviers]

Add calendar option, which follows the selected language and can be overridden manually, to handle non-Gregorian calendars relative times, e.g., "13 months ago" for Chinese calendar.

This needs to be properly defined...

For context, see #6.

[caridy]

We just need to add a new internal slot for [[Calendar]], and the gymnastics to get the option value for that calendar. This should be very straight forward. Volunteer?

[caridy]

@rxaviers I don't think this is solved by #18, If I'm correct, please reopen it.

[FranklinYu]

I agree with @caridy. Please reopen this issue.

[littledan]

It's not clear to me what we should be doing. cc @FrankYFTang

[rxaviers]

There's no notion of date on RelativeTimeFormat as can be seen by the input type being integers (all date manipulations are left to user code). At least until (or whether) a bestFit style is supported.

[littledan]

Right, so it's not clear what @caridy and @FranklinYu are getting at above. Maybe one of them could clarify.

[FranklinYu]

Didn't read through the readme about transition from Date to integer. Sorry for the noise; @littledan @rxaviers please close this again.

[FrankYFTang]

sorry, I don't think it is necessary. The RelativeTimeFormat does not involve calendar at all.
And the example of ""13 months ago" for Chinese calendar." is not a good one since you can also have that in Gregorian calendar.

For example, in current v8 implementation:
out/x64.release/d8
d8> let rtf = new Intl.RelativeTimeFormat("en");
undefined
d8> rtf.format(13, "month")
"in 13 months"
d8> rtf.format(-13, "month")
"13 months ago"