LibJS: Implement Intl.DisplayNames.prototype.of

Note that only option type=region is really implemented. Other types
will resort to the fallback option. This prototype method will be able
to implement other type options once LibUnicode supports more.

Userland/Libraries/LibJS/Tests/builtins/Intl/DisplayNames/DisplayNames.prototype.of.js

@@ -0,0 +1,46 @@
+describe("errors", () => {
+    test("invalid language", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { type: "language" }).of("hello!");
+        }).toThrowWithMessage(RangeError, "'hello!' is not a valid value for option type language");
+    });
+
+    test("invalid region", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { type: "region" }).of("hello!");
+        }).toThrowWithMessage(RangeError, "'hello!' is not a valid value for option type region");
+    });
+
+    test("invalid script", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { type: "script" }).of("hello!");
+        }).toThrowWithMessage(RangeError, "'hello!' is not a valid value for option type script");
+    });
+
+    test("invalid currency", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { type: "currency" }).of("hello!");
+        }).toThrowWithMessage(RangeError, "'hello!' is not a valid value for option type currency");
+    });
+});
+
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Intl.DisplayNames.prototype.of).toHaveLength(1);
+    });
+
+    test("option type region", () => {
+        const en = new Intl.DisplayNames("en", { type: "region" });
+        expect(en.of("US")).toBe("United States");
+
+        const es419 = new Intl.DisplayNames("es-419", { type: "region" });
+        expect(es419.of("US")).toBe("Estados Unidos");
+
+        const zhHant = new Intl.DisplayNames(["zh-Hant"], { type: "region" });
+        expect(zhHant.of("US")).toBe("美國");
+
+        expect(en.of("AA")).toBe("AA");
+        expect(es419.of("AA")).toBe("AA");
+        expect(zhHant.of("AA")).toBe("AA");
+    });
+});