addEventHandler should accept "null" to remove event handlers as per W3C specs

[Apollon77]

see https://html.spec.whatwg.org/multipage/webappapis.html#event-handler-attributes

a listener, which is either null or an event listener responsible for running the event handler processing algorithm. Initially, an event handler's listener must be set to null.

I can provide a PR once #56 is in if wanted

PS: I curently work on https://github.com/project-chip/matter.js and want to get a web-bluetooth compatible BLE handler for Matter commissioning, so that developers can choose if they use it in a browser or node.js :-)

[Apollon77]

@thegecko i can also do a PR then for the types package to get tthat in sync because it is missing there too

[thegecko]

Yes please