Intl.PluralRules should support BigInt

[sffc]

Unclear why it doesn't.

[eemeli]

BigInt support for Intl.NumberFormat was added in #236. I can't find any notes or mentions around then or since about BigInt in Intl.PluralRules, but I do vaguely recall this having been discussed previously on a TG2 call. But probably only discussed.