Fix accidental BigInt-to-Number conversion

This line would accidentally convert the BigInt epoch nanoseconds to
Numbers, potentially losing microseconds or nanoseconds, before
subtracting. It did not affect any results, because no offset
transitions occur on microsecond or nanosecond boundaries.

Author: ptomato

polyfill/lib/ecmascript.mjs

@@ -1455,7 +1455,7 @@ export function InterpretISODateTimeOffset(
   const possibleEpochNs = GetPossibleEpochNanoseconds(timeZone, dt);
   for (let index = 0; index < possibleEpochNs.length; index++) {
     const candidate = possibleEpochNs[index];
-    const candidateOffset = utcEpochNs - candidate;
+    const candidateOffset = utcEpochNs.subtract(candidate).toJSNumber();
     const roundedCandidateOffset = RoundNumberToIncrement(candidateOffset, 60e9, 'halfExpand');
     if (candidateOffset === offsetNs || (matchMinute && roundedCandidateOffset === offsetNs)) {
       return candidate;