It seems like Intl.NumberFormat.prototype.format should support BigDecimal transparently, just as it supports BigInt.

On the ICU level, this should be straightforward, using the same API as BigInt uses.

On a function API level, we have clear precedent with BigInt that we should overload the format and formatToParts methods.

The complexity comes in for options processing: NumberFormat is based on rounding the input, but BigDecimal is all about avoiding implicit rounding. How should that all be handled?