fix filename, export helper types/interfaces

[imirkin]

I also noticed that the typings didn't actually make it to the npm package. Hopefully this package.json change will make that happen, but I'm no npm pro.

I'm also not 100% sure what the deal is with default exports vs named exports, but the js module produces named exports - I think the two ought to match.

With this, I can do

import {BigDecimal} from "@yaffle/bigdecimal"

in my ts setup.

[Yaffle]

I have updated npm package, thanks