LibJS: Fix Object::delete_property() with numeric string property

- We have to check if the property name is a string before calling
  as_string() on it
- We can't as_number() the same property name but have to use the parsed
  index number

Fixes #3950.

Libraries/LibJS/Tests/operators/delete-basic.js

@@ -46,6 +46,9 @@ test("deleting array indices", () => {
     expect(a.hasOwnProperty(1)).toBeFalse();
     expect(a.hasOwnProperty(2)).toBeFalse();
     expect(Object.getOwnPropertyNames(a)).toHaveLength(1);
+
+    expect(delete a["42"]).toBeTrue();
+    expect(Object.getOwnPropertyNames(a)).toHaveLength(1);
 });
 
 test("deleting non-configurable property", () => {