Context

The NewDecimal128 constructor currently accepts any 128-bit pattern and produces a Decimal128 object that could be marshaled into invalid extended JSON. This can be rather confusing to users. For example: https://go.dev/play/p/FgV82UGH3i9

Definition of done

Update NewDecimal128 to return an error if an illegal pattern would be produced by the user's h/l input.

Pitfalls

While this will help users avoid marshaling extended JSON that can't be parsed, it doesn't avoid the problem entirely. A user could get around this validation by creating the serialization directly.