LibJS: Implement Intl.supportedValuesOf

This is a stage 3 ECMA-402 proposal:
https://tc39.es/proposal-intl-enumeration/

Userland/Libraries/LibJS/Tests/builtins/Intl/Intl.supportedValuesOf.js

@@ -0,0 +1,63 @@
+describe("errors", () => {
+    test("invalid key", () => {
+        expect(() => {
+            Intl.supportedValuesOf("hello!");
+        }).toThrowWithMessage(RangeError, "hello! is not a valid key");
+    });
+});
+
+describe("normal behavior", () => {
+    const isSorted = array => {
+        return array.slice(1).every((item, i) => array[i] <= item);
+    };
+
+    test("length is 1", () => {
+        expect(Intl.supportedValuesOf).toHaveLength(1);
+    });
+
+    test("calendar", () => {
+        const values = Intl.supportedValuesOf("calendar");
+        expect(isSorted(values)).toBeTrue();
+
+        expect(values.indexOf("gregory")).not.toBe(-1);
+    });
+
+    test("collation", () => {
+        const values = Intl.supportedValuesOf("collation");
+        expect(isSorted(values)).toBeTrue();
+
+        expect(values.indexOf("default")).not.toBe(-1);
+    });
+
+    test("currency", () => {
+        const values = Intl.supportedValuesOf("currency");
+        expect(isSorted(values)).toBeTrue();
+
+        expect(values.indexOf("USD")).not.toBe(-1);
+        expect(values.indexOf("XXX")).not.toBe(-1);
+    });
+
+    test("numberingSystem", () => {
+        const values = Intl.supportedValuesOf("numberingSystem");
+        expect(isSorted(values)).toBeTrue();
+
+        expect(values.indexOf("latn")).not.toBe(-1);
+        expect(values.indexOf("arab")).not.toBe(-1);
+    });
+
+    test("timeZone", () => {
+        const values = Intl.supportedValuesOf("timeZone");
+        expect(isSorted(values)).toBeTrue();
+
+        expect(values.indexOf("UTC")).not.toBe(-1);
+        expect(values.indexOf("America/New_York")).not.toBe(-1);
+    });
+
+    test("unit", () => {
+        const values = Intl.supportedValuesOf("unit");
+        expect(isSorted(values)).toBeTrue();
+
+        expect(values.indexOf("acre")).not.toBe(-1);
+        expect(values.indexOf("fluid-ounce")).not.toBe(-1);
+    });
+});