Adding shippingMark, allowancesCharges; make validTo optional on commercial invoice
- Add shippingMark header field (uncefact:marksAndNumbers) for linking documents to physical goods
- Add allowancesCharges array at invoice level, positioned after line items and before totalAmount (uncefact:specifiedTradeAllowanceCharge)
- Remove validTo from required fields; invoices do not expire in the conventional sense — validTo represents VC credential validity, not invoice expiry
- Remove validTo from example to suppress rendering of the field by default
Addresses feedback from issue #25 (closed) (Kokichi Watanabe).
Closes #25 (closed)