LibJS: Implement Intl.Locale.prototype.firstDayOfWeek

This is a normative change in the Intl Locale Info spec. See:
f03a814

Userland/Libraries/LibJS/Tests/builtins/Intl/Locale/Locale.prototype.firstDayOfWeek.js

@@ -0,0 +1,44 @@
+describe("errors", () => {
+    test("called on non-Locale object", () => {
+        expect(() => {
+            Intl.Locale.prototype.firstDayOfWeek;
+        }).toThrowWithMessage(TypeError, "Not an object of type Intl.Locale");
+    });
+
+    test("invalid options", () => {
+        [100, Infinity, NaN, "hello", 152n, true].forEach(value => {
+            expect(() => {
+                new Intl.Locale("en", { firstDayOfWeek: value }).firstDayOfWeek;
+            }).toThrowWithMessage(
+                RangeError,
+                `${value} is not a valid value for option firstDayOfWeek`
+            );
+        });
+    });
+
+    // FIXME: Spec issue: It is not yet clear if the following invalid values should throw. For now, this tests our
+    //        existing workaround behavior, which returns "undefined" for invalid extensions.
+    //        https://github.com/tc39/proposal-intl-locale-info/issues/78
+    test("invalid extensions", () => {
+        [100, Infinity, NaN, "hello", 152n, true].forEach(value => {
+            expect(new Intl.Locale(`en-u-fw-${value}`).firstDayOfWeek).toBeUndefined();
+        });
+    });
+});
+
+describe("normal behavior", () => {
+    test("valid options", () => {
+        expect(new Intl.Locale("en").firstDayOfWeek).toBeUndefined();
+
+        ["mon", "tue", "wed", "thu", "fri", "sat", "sun"].forEach((day, index) => {
+            expect(new Intl.Locale(`en-u-fw-${day}`).firstDayOfWeek).toBe(index + 1);
+            expect(new Intl.Locale("en", { firstDayOfWeek: day }).firstDayOfWeek).toBe(index + 1);
+            expect(new Intl.Locale("en", { firstDayOfWeek: index + 1 }).firstDayOfWeek).toBe(
+                index + 1
+            );
+            expect(new Intl.Locale("en-u-fw-mon", { firstDayOfWeek: day }).firstDayOfWeek).toBe(
+                index + 1
+            );
+        });
+    });
+});