LibJS: Ensure relevant extension keys are included in ICU locale data

This is a normative change in the ECMA-402 spec. See:
7508197

In our implementation, we don't have the affected AOs directly, as we
delegate to ICU. So instead, we must ensure we provide ICU a locale with
the relevant extension keys present.

Libraries/LibJS/Tests/builtins/Intl/RelativeTimeFormat/RelativeTimeFormat.prototype.format.js

@@ -129,6 +129,11 @@ describe("second", () => {
 
         runTest("second", "narrow", "auto", en, ar, pl);
     });
+
+    test("numberingSystem set via locale options", () => {
+        const formatter = new Intl.RelativeTimeFormat("en", { numberingSystem: "arab" });
+        expect(formatter.format(1, "second")).toBe("in ١ second");
+    });
 });
 
 describe("minute", () => {