Subfeature for responding to onMessage by Promise (#2927)
Subfeature for responding to onMessage by Promise (#2927)

* Subfeature for responding to onMessage by Promise

In Firefox, you can return a Promise instead of calling `sendResponse` in a messaging listener. This is also documented on the respective event pages, but not part of the compatibility info yet.

* Fix invalid JSON formatting in onMessage

* Adjust onMessage(External) respond with Promise

Mark opera and edge as not supporting responding by Promise.
Wrap "Promise" in a code block in the description.

* Add missing comma

* Add another missing comma

mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data