Add support for scoped hotkey

[khiga8]

What

Add support for scoped hotkeys when the data-hotkey-scope attribute is specified.

For example:

<!-- button click can only be triggered when hotkey is pressed from textarea#text-area-1 -->
<button data-hotkey="Control+P" data-hotkey-scope="text-area-1"> Button 1 </button>
<textarea id="text-area-1"> </textarea>
<textarea id="text-area-2"> </textarea>

Why

Currently github/hotkey only supports global hotkeys but increasingly we are seeing a need for hotkey actions that can be triggered from a textarea. Rather than relying on custom hotkey implementations for scoped shortcuts, we should expand this library.

Co-authored-by: @keithamus

[khiga8]

paired with @keithamus!

Ready for another review 🚀