LibJS: Implement a nearly empty Intl.PluralRules object

This adds plumbing for the Intl.PluralRules object, constructor, and
prototype.

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

@@ -0,0 +1,13 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            Intl.PluralRules();
+        }).toThrowWithMessage(TypeError, "Intl.PluralRules constructor must be called with 'new'");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 0", () => {
+        expect(Intl.PluralRules).toHaveLength(0);
+    });
+});