[@mantine/modals] Fixed closeModal async logic

[Njiallu]

Closes #3289

Made the logic simpler and not assume that the closed modal is the last in the stack.

[rtivital]

Thanks!

[D3SOX]

When trying to close a modal like this

const submit = () => ctx.closeModal(id);
const id = ctx.openModal(
  ...
  children: <Button onClick={submit}>Submit</Button>
);

I get an Uncaught TypeError: modal is undefined

[cyantree]

@D3SOX FYI, I've taken another shot at this and also investigated your reported behaviour: #3498 (comment)