LibJS: Implement Temporal.Now.plainDate()

...and ten required AOs we didn't have yet:

- BalanceISODate
- BalanceISODateTime
- BalanceISOYearMonth
- BalanceTime
- BuiltinTimeZoneGetPlainDateTimeFor
- GetISOPartsFromEpoch
- GetOffsetNanosecondsFor
- ParseTemporalTimeZone
- SystemDateTime
- ToTemporalTimeZone

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

@@ -0,0 +1,27 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Now.plainDate).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        const plainDate = Temporal.Now.plainDate(calendar);
+        expect(plainDate).toBeInstanceOf(Temporal.PlainDate);
+        expect(plainDate.calendar).toBe(calendar);
+    });
+
+    test("custom time zone", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        const timeZone = {
+            getOffsetNanosecondsFor() {
+                return 86400000000000;
+            },
+        };
+        const plainDate = Temporal.Now.plainDate(calendar);
+        const plainDateWithOffset = Temporal.Now.plainDate(calendar, timeZone);
+        // Yes, this will fail if a day, month, or year change happens between the above two lines :^)
+        expect(plainDateWithOffset.year).toBe(plainDate.year);
+        expect(plainDateWithOffset.month).toBe(plainDate.month);
+        expect(plainDateWithOffset.day).toBe(plainDate.day + 1);
+    });
+});