Closes the current interchange by writing its footer segment.
async x12writer.createInterchangeFooter(): Promise<void>
The CreateInterchangeFooter method will write any existing segments to the output stream and then generate and write the interchange closing segment (IEA or UNZ). If there is an open transaction set, CreateInterchangeFooter will first call CreateTransactionFooter to close it. If there is an open functional group, CreateInterchangeFooter will first call CreateFunctionalGroupFooter to close it. If there is no interchange open, an exception will be thrown.