Fix `v8::OwnedIsolate` memory leak caused by retain cycle in MiniV8::create_function

[AlaaAlHallaq]

The only way to test this at least for my current expertise in Rust :

• is by adding a print statement in drop impl for OwnedIsolate or a break point. the following example:

  let ctx = MiniV8::new();
  let obj = ctx.create_object();
  let fc = ctx.create_function(|env| {
      let me = env
          .this
          .as_object()
          .ok_or(mini_v8::Error::FromJsConversionError {
              from: "value",
              to: "object",
          })?;
      let v: Value = me.get("name")?;
      return Ok(v);
  });
  obj.set("name", "jack").unwrap();
  obj.set("getName", fc).unwrap();
  ctx.global().set("person", obj).unwrap();
  let returned_name: String = ctx.eval("person.getName()").unwrap();
  println!("person.getName() = {:?}", returned_name);
  drop(ctx);

  before this apply this pr nether the print statement nor the break point will be reached & v8::OwnedIsolate wont be dropped. after applying it, it just works.