feat: add initial focus

[LukasHirt]

We've added an initial focus so that when the File picker is mounted, it immediately becomes focused.

[pascalwengerter]

LGTM 🎯

[kulmann]

👍

[VP-]

This does not work when wrapped in a web component. According to the source, mounted() is called before the element is added to the DOM. A focus() does nothing at that point.

Also, if the primary use case of the file picker is a widget, then it should not steal the focus on its own. Instead, the web component should properly handle focus() calls from outside. This would allow the container to decide whether and when the file picker gets the focus.

Handling focus() calls is required anyway to allow proper focus-trapping when used in a modal dialog: Since the dialog might contain other elements, the file picker can't trap the focus only inside itself. So the container has to do it. And since the file picker is typically the first element, then it will need to handle external calls to focus() by forwarding them to the first focusable element inside itself.