LibJS: Implement WeakMap changes from 'Symbol as WeakMap Keys Proposal'

Author: https://github.com/IdanHo
Commit: a80d3fdf49
Pull-request: https://github.com/SerenityOS/serenity/pull/14361
Reviewed-by: https://github.com/alimpfard ✅
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Tests/builtins/WeakMap/WeakMap.js

@@ -37,6 +37,6 @@ describe("regressions", () => {
     test("missing key/value properties on iterable entry", () => {
         expect(() => {
             new WeakMap([{}]);
-        }).toThrowWithMessage(TypeError, "undefined is not an object");
+        }).toThrowWithMessage(TypeError, "undefined cannot be held weakly");
     });
 });