fix: format pre-epoch Xero dates

[ryanduguid]

Summary

• support negative Unix-millisecond values in Xero's /Date(...)/ strings
• accept an optional numeric UTC offset suffix
• add a regression test for a pre-1970 date

The prior formatter only matched unsigned timestamps followed by a positive offset, leaving valid pre-epoch Xero dates unformatted.

Testing

• npm test (131 passed)
• npm run build