Hotkeys are fired when trying to type into a text field inside a custom element

When typing in an input (textarea, input tag, editable, …) which is itself placed inside of a custom html element, the hotkey is still fired. That's because the [isFormField](https://github.com/github/hotkey/blob/main/src/utils.ts#L4) handler checks only the target element, which is this case is the custom element. It should instead use the `explicitOriginalTarget` property, since that contains the actual input element.

Here's a minimal reproduction: https://jsfiddle.net/zm7etdh3/