useHotKeys: support a hotkey that is just a modifier key like Alt

[p-bakker]

In out current application implemented using other technology we have a keyboard shortcut defined for just the Alt key to toggle something in the UI (between a List view and a Detail view, but that's not really relevant)

Mantine's useHotKeys doesn't seem to support that, hence this FR

Note that implementing this requires preventing conflicts with other hotkeys.
If there are for example 2 hotkeys defined, one for alt and one for alt+x, it shouldn't fire the action for alt on the keyDown event of the Alt key, but it should detect that Alt was pressed down, but that there's the potential that the user is going to press the x next.
If that happens, the call the alt+x action
If the Alt key is released without the x key being pressed, then call the alt action