Add number formatting utilities

From an offline discussion:

• formatNumber(): use Intl.formatNumber()
• formatTokenAmount(): build on top of formatNumber() and add some options to make it easier to format token amounts (e.g. see this function)

[bpierre]

@ECWireless I think we could adapt them to make it a bit simpler to use:

pow, multiplier and decimals

I think we could replace these three params by a single decimals = 18, since it is both defined by ERC-20 and widely used. Similarly to amount, this should be either a number, or any object with a toNumber() method (i.e. we should call Number() on it).

amount

This should be either a string, a BigInt instance, or any object with a .toString() method (i.e. we should call String() on it).

rounding

I think we could rename this to something like displayDecimals, roundToDecimals, decimalPlaces, or just precision?

isIncoming

I don’t think this is needed, we can use a sign on the BigInt + displaySign (or signDisplay) to determine that.

displaySign

We might want to name it signDisplay to align it with the new Intl.NumberFormat API.

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@sohkai @ECWireless what would you guys think about naming the function formatAmount()? It might sound a bit generic, but I think there is enough context for authors to know what it is about. It would also make it clearer that we can use it to format currencies like USD as well.

[sohkai]

Closed with #728