Reset document.title on component unmount

[ottomated]

Describe the problem

I expect this example to work. That is, when a component is unmounted that contains the following code:

<svelte:head>
  <title>Title</title>
</svelte:head>

it should reset the document.title to the previous value.

This is particularly relevant when using shallow routing, where rendering a page component manually is recommended.

Describe the proposed solution

Something like

$.head(($$anchor) => {
  const previous_title = $.document.title;
  $.teardown(() => {
    $.document.title = previous_title;
  });
  $.document.title = `title`;
});

I'm not sure whether there would need to be some global queue of titles, i.e.

const title_queue = [];
function set_title(title) {
  render_effect(() => {
    title_queue.push(document.title);
    document.title = title;
    return () => {
      if (title_queue.length === 0) return;
      document.title = title_queue.pop();
    };
  });
}

Importance

would make my life easier

[Rich-Harris]

dupe of #7656, will be fixed by #14116