LibJS: Implement the Intl.DisplayNames constructor

There is notably FIXME notations in this commit regarding Unicode locale
extensions. We are not parsing extensions (or private use extensions) at
all yet.

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

@@ -4,10 +4,54 @@ describe("errors", () => {
             Intl.DisplayNames();
         }).toThrowWithMessage(TypeError, "Intl.DisplayNames constructor must be called with 'new'");
     });
+
+    test("options is undefined", () => {
+        expect(() => {
+            new Intl.DisplayNames("en");
+        }).toThrowWithMessage(TypeError, "options is undefined");
+    });
+
+    test("options is an invalid type", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", true);
+        }).toThrowWithMessage(TypeError, "Options is not an object");
+    });
+
+    test("style option is invalid ", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { style: "hello!" });
+        }).toThrowWithMessage(RangeError, "hello! is not a valid value for option style");
+    });
+
+    test("type option is invalid ", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { type: "hello!" });
+        }).toThrowWithMessage(RangeError, "hello! is not a valid value for option type");
+    });
+
+    test("fallback option is invalid ", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", { type: "region", fallback: "hello!" });
+        }).toThrowWithMessage(RangeError, "hello! is not a valid value for option fallback");
+    });
+
+    test("missing type options ", () => {
+        expect(() => {
+            new Intl.DisplayNames("en", {});
+        }).toThrowWithMessage(TypeError, "options.type is undefined");
+    });
 });
 
 describe("normal behavior", () => {
     test("length is 2", () => {
         expect(Intl.DisplayNames).toHaveLength(2);
     });
+
+    test("all valid types", () => {
+        ["language", "region", "script", "currency"].forEach(type => {
+            expect(() => {
+                new Intl.DisplayNames("en", { type: type });
+            }).not.toThrow();
+        });
+    });
 });