LibJS: Implement the ECMA-402 definition of DefaultTimeZone

Simply defer to LibTimeZone to retrieve the system's current time zone.
Also update some Temporal tests to explicitly set the time zone to UTC.

Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.cpp

@@ -56,11 +56,11 @@ String canonicalize_time_zone_name(String const& time_zone)
 }
 
 // 11.1.3 DefaultTimeZone ( ), https://tc39.es/proposal-temporal/#sec-defaulttimezone
-// NOTE: This is the minimum implementation of DefaultTimeZone, supporting only the "UTC" time zone.
+// 15.1.3 DefaultTimeZone ( ), https://tc39.es/proposal-temporal/#sup-defaulttimezone
 String default_time_zone()
 {
-    // 1. Return "UTC".
-    return "UTC";
+    // The DefaultTimeZone abstract operation returns a String value representing the valid (11.1.1) and canonicalized (11.1.2) time zone name for the host environment's current time zone.
+    return ::TimeZone::current_time_zone();
 }
 
 // 11.6.1 ParseTemporalTimeZone ( string ), https://tc39.es/proposal-temporal/#sec-temporal-parsetemporaltimezone

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainDate.js

@@ -17,7 +17,7 @@ describe("correct behavior", () => {
                 return 86400000000000;
             },
         };
-        const plainDate = Temporal.Now.plainDate(calendar);
+        const plainDate = Temporal.Now.plainDate(calendar, "UTC");
         const plainDateWithOffset = Temporal.Now.plainDate(calendar, timeZone);
         if (plainDate.dayOfYear === plainDate.daysInYear) {
             expect(plainDateWithOffset.year).toBe(plainDate.year + 1);

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainDateISO.js

@@ -15,7 +15,7 @@ describe("correct behavior", () => {
                 return 86400000000000;
             },
         };
-        const plainDate = Temporal.Now.plainDateISO();
+        const plainDate = Temporal.Now.plainDateISO("UTC");
         const plainDateWithOffset = Temporal.Now.plainDateISO(timeZone);
         if (plainDate.dayOfYear === plainDate.daysInYear) {
             expect(plainDateWithOffset.year).toBe(plainDate.year + 1);

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainDateTime.js

@@ -37,7 +37,7 @@ describe("correct behavior", () => {
             },
         };
 
-        const plainDateTime = Temporal.Now.plainDateTime(calendar);
+        const plainDateTime = Temporal.Now.plainDateTime(calendar, "UTC");
         const plainDateTimeWithOffset = Temporal.Now.plainDateTime(calendar, timeZone);
 
         if (plainDateTime.year !== plainDateTimeWithOffset.year) return;
@@ -58,7 +58,7 @@ describe("correct behavior", () => {
             },
         };
 
-        const plainDateTime = Temporal.Now.plainDateTime(calendar);
+        const plainDateTime = Temporal.Now.plainDateTime(calendar, "UTC");
         const plainDateTimeWithOffset = Temporal.Now.plainDateTime(calendar, timeZone);
 
         if (plainDateTime.year !== plainDateTimeWithOffset.year) return;

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainDateTimeISO.js

@@ -35,7 +35,7 @@ describe("correct behavior", () => {
             },
         };
 
-        const plainDateTime = Temporal.Now.plainDateTimeISO();
+        const plainDateTime = Temporal.Now.plainDateTimeISO("UTC");
         const plainDateTimeWithOffset = Temporal.Now.plainDateTimeISO(timeZone);
 
         if (plainDateTime.year !== plainDateTimeWithOffset.year) return;
@@ -55,7 +55,7 @@ describe("correct behavior", () => {
             },
         };
 
-        const plainDateTime = Temporal.Now.plainDateTimeISO();
+        const plainDateTime = Temporal.Now.plainDateTimeISO("UTC");
         const plainDateTimeWithOffset = Temporal.Now.plainDateTimeISO(timeZone);
 
         if (plainDateTime.year !== plainDateTimeWithOffset.year) return;

Userland/Libraries/LibJS/Tests/builtins/Temporal/Now/Now.plainTimeISO.js

@@ -15,7 +15,7 @@ describe("correct behavior", () => {
                 return 86400000000000;
             },
         };
-        const plainTime = Temporal.Now.plainTimeISO();
+        const plainTime = Temporal.Now.plainTimeISO("UTC");
         const plainTimeWithOffset = Temporal.Now.plainTimeISO(timeZone);
         // FIXME: Compare these in a sensible way
     });