added sendResponse argument to browser.runtime.onMessage.addEventListener()

[eugbyte]

In the library, browser.runtime.onMessage.addEventListener(message, sender) only accepts 2 arguments. However, in the official Javascript API, this method receives an additional 3rd argument, sendResponse(), a callback function.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage

[Lusito]

Sorry about the work you've put into this, but I won't merge this.
As you can see in the page you linked: "Returning a Promise is now preferred, as sendResponse() will be removed from the W3C spec."

Please use a promise as return value instead of using sendResponse.

[montogeek]

Does this polyfill cover returning a Promise?
It is currently not supported https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage

[eugbyte]

@montogeek
#54