add TypeConstraint for supported currencies

[sherrardb]

i don't necessarily like the added maintenance of this list, but i do like shifting the error handling locally vs an unnecessary API call.

Error: invalid_request_error - Invalid currency: zzz. Stripe currently supports these currencies: usd, aed, afn, all, amd, ang, aoa, ars, aud, awg, azn, bam, bbd, bdt, bgn, bif, bmd, bnd, bob,
 brl, bsd, bwp, bzd, cad, cdf, chf, clp, cny, cop, crc, cve, czk, djf, dkk, dop, dzd, egp, etb, eur, fjd, fkp, gbp, gel, gip, gmd, gnf, gtq, gyd, hkd, hnl, hrk, htg, huf, idr, ils, inr, isk, jmd, jpy, kes, kgs, khr, kmf, krw, kyd, kzt, lak, lbp, lkr, lrd, lsl, mad, mdl, mga, mkd, mmk, mnt, mop, mro, mur, mvr, mwk, mxn, myr, mzn, nad, ngn, nio, nok, npr, nzd, pab, pen, pgk, php, pkr, pln, pyg, qar, ron, rsd, rub, rwf, sar, sbd, scr, sek, sgd, shp, sll, sos, srd, std, szl, thb, tjs, top, try, ttd, twd, tzs, uah, ugx, uyu, uzs, vnd, vuv, wst, xaf, xcd, xof, xpf, yer, zar, zmw, eek, lvl, svc, vef, ltl On parameter: currency