Fix timezone in formal string conversion

[l0drex]

Whenever the offset minutes are 0, the timezone was set to "Z" due to using or instead of and.

[l0drex]

Here is a workaround in typescript:

function toFormalString(simple: Simple): string {
  let formalString = simple.toFormalString();
  
  if (formalString.endsWith("Z")){
    // remove the z, will be added in the next step if applicable
    formalString = formalString.substring(0, formalString.length - 1);

    if (simple.getHours() && (simple.getTZHours() !== 0 || simple.getTZMinutes() !== 0)) {
      formalString += getOffset(simple);
    }
  }

  return formalString;
}

function getOffset(date: Simple): string {
  let hours = date.getTZHours() ?? 0;
  let minutes = date.getTZMinutes();

  if (date.getTZHours() === 0 && date.getTZMinutes() === 0) return "Z"

  let offset = (hours >= 0 ? "+" : "-") + Math.abs(hours).toString().padStart(2, "0");

  if (minutes != null) {
    offset += ':' + Math.abs(minutes).toString().padStart(2, "0");
  }

  return offset;
}