Add unit attribute to the formatToParts output to match Intl.RelativeTimeFormat

[FrankYFTang]

let nf = new Intl.NumberFormat("en", {style: "unit", unit: "day"})
nf.format(3.4)
> "3.4 days"
nf.formatToParts(3.4)
>0: {type: "integer", value: "3"}
1: {type: "decimal", value: "."}
2: {type: "fraction", value: "4"}
3: {type: "literal", value: " "}
4: {type: "unit", value: "days"}

let rf = new Intl.RelativeTimeFormat("en")
rf.format(3.4, "day")
> "in 3.4 days"
rf.formatToParts(3.4, "day")
> 0: {type: "literal", value: "in "}
1: {type: "integer", value: "3", unit: "day"}
2: {type: "decimal", value: ".", unit: "day"}
3: {type: "fraction", value: "4", unit: "day"}
4: {type: "literal", value: " days"}

Notice the output from the Intl.RelativeTimeFormat has the unit: "day" in the integer/decimal/fraction
Should we also make the Intl.NumberFormat to output that ?

I think we should change the spec in NumberFormat v3 to make Intl.NumberFormat output

nf.formatToParts(3.4)
>0: {type: "integer", value: "3", unit: "day"}
1: {type: "decimal", value: ".", unit: "day"}
2: {type: "fraction", value: "4", unit: "day"}
3: {type: "literal", value: " "}
4: {type: "unit", value: "days"}

There is a second issue of the 4: {type: "literal", value: " days"} from the Intl.RelativeTimeFormat, which I think we should discuss in a ECMA402 issue.

[sffc]

I think it would be great to do this, but ICU doesn't support this right now, and I don't want to block NFv3 on this feature, because I want NFv3 to ship with the current feature set based on ICU 69. I would rather consider it as an enhancement in a future proposal.

[FrankYFTang]

or maybe we should do

nf.formatToParts(3.4)
>0: {type: "integer", value: "3", unit: "day"}
1: {type: "decimal", value: ".", unit: "day"}
2: {type: "fraction", value: "4", unit: "day"}
3: {type: "literal", value: " ", unit: "day"}
4: {type: "unit", value: "days", unit: "day"}

to make it consistent with the future Intl.DurationFormat when there maybe multiple PARTS of different units in the output

[FrankYFTang]

It won't require any ICU change to do

nf.formatToParts(3.4)
>0: {type: "integer", value: "3", unit: "day"}
1: {type: "decimal", value: ".", unit: "day"}
2: {type: "fraction", value: "4", unit: "day"}
3: {type: "literal", value: " ", unit: "day"}
4: {type: "unit", value: "days", unit: "day"}

we already know from the constructor the unit is day, and we just attach to all output nodes. Require no new ICU feature to implement that.

[sffc]

This feature request largely fell through the cracks. I think this can still be added as a small normative PR against the spec after Stage 4. I will move this issue to the ecma402 repository after the proposal is merged in.