v2 - Allow modification of rounding mode when calling transformScale

[tranhl]

Related to #513

My team and I are currently integrating with a payments processor that requires minor units in the request. The documentation for v2 suggests using toSnapshot to grab the amount in minor units, but this doesn't work when the scale for the Dinero object is greater than the currency's exponent.

This is where transformScale comes in (or so we thought). Right now, transformScale always truncates precision when going from a larger scale to a smaller scale.

I suggest allowing us to provide a rounding mode (e.g. halfUp) when calling transformScale to allow flexibility in how we manage the loss of precision.

For example, this is how this new feature could look like in code:

const d = dinero({ amount: 12345, currency: USD, scale: 3 })
const scaled = transformScale(d, { newScale: d.toJSON().currency.exponent, round: halfUp })

console.log(scaled.toJSON()) // { amount: 1235, scale: 2, currency: ... }

Note how the scaled Dinero's amount is 1235 instead of 1234, which is what we would get with current behavior.

[sarahdayan]

Hey @tranhl, I think this would be awesome to provide a way to handle how transformScale handles lossy transformations.

Do you want to take a stab at it?

If you do, I'd recommend branching off of #309 which changes a bunch of things in a breaking way. I'm expecting to merge it soon, so this would be safer.

[sarahdayan]

I had forgotten that #309 actually handles it by letting you pass a custom divide function (see preview docs).