Rename currency fields subunits -> major units

[diehuxx]

Update currency amount representation and validation based on this spec change: TBD54566975/tbdex#206

Changes based on the spec

Update the protocol spec, test-vectors, and examples to represent currencies as decided in issue #199 in this comment here.

1. Strings
2. Containing major unit amounts
3. Decimals using period . as the decimal separator regardless of currency.
   Currency representation tbdex#199 (comment)

The updated fields taken from this comment here.

Old	New
offering.payinCurrency.minSubunits	min or minAmount
offering.payinCurrency.maxSubunits	max or maxAmount
offering.payoutCurrency.minSubunits	min or minAmount
offering.payoutCurrency.maxSubunits	max or maxAmount
offering.paymentMethods[*].feeSubunits	fee
rfq.payinSubunits	amount or payinAmount
quote.payin.amountSubunits	amount
quote.payin.feeSubunits	fee
quote.payout.amountSubunits	amount
quote.payout.feeSubunits	fee

Notes

This PR does NOT validate that currency amount strings are decimals. That will come in a later PR which validates TBDex messages/resources against the published JSON schema. tbdex-js already does this.