LibJS: Fix ProxyObject get/set with symbol property name

We can't assume that property names can be converted to strings anymore,
as we have symbols. Use name.to_value() instead.

This makes something like this possible:

    new Proxy(Object, { get(t, p) { return t[p] }  })[Symbol.hasInstance]

Libraries/LibJS/Tests/builtins/Proxy/Proxy.handler-get.js

@@ -38,6 +38,7 @@ describe("[[Get]] trap normal behavior", () => {
         expect(p.baz).toBe(3);
         expect(p.qux).toBe(3);
         expect(p.test).toBeUndefined();
+        expect(p[Symbol.hasInstance]).toBeUndefined();
     });
 });