LibJS: Correcly handle surrogates in escape()

Fixes test/annexB/built-ins/escape/escape-above{,-astral}.js in
test262. All tests in test/annexB/built-ins/escape pass now.

Userland/Libraries/LibJS/Tests/builtins/functions/escapeUnescape.js

@@ -4,6 +4,7 @@ test("escape", () => {
         ["äöü", "%E4%F6%FC"],
         ["ć", "%u0107"],
         ["@*_+-./", "@*_+-./"],
+        ["\ud834\udf06", "%uD834%uDF06"],
     ].forEach(test => {
         expect(escape(test[0])).toBe(test[1]);
     });