ensure addListener takes a synchronous callback

[max-baz]

Another potential improvement to help with #404

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

I'm reusing the same pattern used in:

browserpass-extension/src/background.js

Lines 61 to 67 in 0138231

	// handle incoming messages
	chrome.runtime.onMessage.addListener(function (message, sender, sendResponse) {
	receiveMessage(message, sender, sendResponse);
	// allow async responses after this function returns
	return true;
	});