allow directly passing in HTMLElement to default ui element option

[stefanprobst]

currently, the default pagefind ui expects a query selector string as options.element, see https://github.com/CloudCannon/pagefind/blob/main/pagefind_ui/default/ui-core.js#L40

it would be nice if it were possible to directly pass in a reference to an html element as well, i.e.:

- const dom = document.querySelector(selector);
+ const dom = selector instanceof HTMLElement ? selector : document.querySelector(selector);

would you accept a pr for this?

[bglw]

I would certainly accept a PR for this 🙂