LibJS: Compare Unicode locale variants using case-insensitive matching

In the IsStructurallyValidLanguageTag AO, we of course cannot assume the
variants are canonicalized to lower-case yet, because canonicalization
hasn't happened yet.

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

@@ -18,6 +18,13 @@ describe("errors", () => {
     });
 
     test("duplicate variant subtags", () => {
+        expect(() => {
+            Intl.getCanonicalLocales("en-posix-POSIX");
+        }).toThrowWithMessage(
+            RangeError,
+            "en-posix-POSIX is not a structurally valid language tag"
+        );
+
         expect(() => {
             Intl.getCanonicalLocales("en-POSIX-POSIX");
         }).toThrowWithMessage(