LibJS: Make parse_simplified_iso8601() use Optional<int> instead of -1

...for the non-success state. This fixes a crash when parsing dates with
the year -1, as we would assert successful parsing ("year != -1").
Mixing Optional and -1 seems worse and more complicated than just using
Optional for all the values, so I did that instead.

Userland/Libraries/LibJS/Tests/builtins/Date/Date.parse.js

@@ -10,6 +10,7 @@ test("basic functionality", () => {
     expect(Date.parse("2020T23:59Z")).toBe(1577923140000);
 
     expect(Date.parse("+020000")).toBe(568971820800000);
+    expect(Date.parse("-000001")).toBe(-62198755200000);
     expect(Date.parse("+020000-01")).toBe(568971820800000);
     expect(Date.parse("+020000-01T00:00:00.000Z")).toBe(568971820800000);