LibJS: Handle negativity in Temporal's ApplyUnsignedRoundingMode

I don't fully understand the BigInt math here, as the computation for
d1 and d2 don't align with the spec due to BigInt logic. This was
discussed a bit in SerenityOS's Discord some years ago:

https://discord.com/channels/830522505605283862/851522357734408232/978786665306918932

But some new tests in test262 indicate that we need to handle negative
values here, instead of just throwing away the sign.

Libraries/LibJS/Runtime/Temporal/AbstractOperations.cpp

@@ -947,10 +947,11 @@ Crypto::SignedBigInteger apply_unsigned_rounding_mode(Crypto::SignedDivisionResu
         return r2;
 
     // 6. Let d1 be x – r1.
-    auto d1 = x.remainder.unsigned_value();
+    auto const& d1 = x.remainder;
 
     // 7. Let d2 be r2 – x.
-    auto d2 = MUST(increment.minus(x.remainder.unsigned_value()));
+    auto d2 = x.remainder.is_negative() ? x.remainder.plus(increment) : x.remainder.minus(increment);
+    d2.negate();
 
     // 8. If d1 < d2, return r1.
     if (d1 < d2)

Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.toString.js

@@ -52,6 +52,22 @@ describe("correct behavior", () => {
             expect(instant.toString(options)).toBe(expected);
         }
     });
+
+    test("rounding", () => {
+        const instant = new Temporal.Instant(-999999999999999990n);
+        const roundedDown = "1938-04-24T22:13:20.000Z";
+        const roundedUp = "1938-04-24T22:13:20.001Z";
+
+        for (const roundingMode of ["halfCeil", "halfFloor", "halfExpand", "halfTrunc", "halfEven", "floor", "trunc"]) {
+            const options = { smallestUnit: "millisecond", roundingMode };
+            expect(instant.toString(options)).toBe(roundedDown);
+        }
+
+        for (const roundingMode of ["ceil", "expand"]) {
+            const options = { smallestUnit: "millisecond", roundingMode };
+            expect(instant.toString(options)).toBe(roundedUp);
+        }
+    });
 });
 
 describe("errors", () => {