Enable other extensions to activate ff2mpv

[arthsmn]

Is your feature request related to a problem? Please describe.
I would like to integrate ff2mpv with vimium c, as requested here. But it seems that ff2mpv doesn't have the interface to allow other extensions to activate it.

Describe the solution you'd like
A way that extensions could activate ff2mpv.

[woodruffw]

I'm not necessarily opposed to this, but I don't know how to do it. If someone were to send a patch and an explanation of how it works, I'd be happy to merge it and include it in the next release.

[DanSM-5]

I just recently added some changes in the code for migrating to mv3 that could be used for this. I'm talking specifically for the listener for runtime.onMessage https://github.com/eNV25/ff2mpv/pull/1/files#diff-d59a73ad52dc1dc2b2d687747f1225e38b14324cbd2fa3dd73b2be2b35e42d35R96.

According to the documentation runtime.sendMessage can pass a extensionId to call another extension (ff2mpv in this case). It would be a matter to support a new type of message e.g. openVideo with the url as the payload of that message from the extension that wants to call mpv.

// External extension
chrome.runtime.sendMessage(ff2mpvExtensionId, { type: 'openVideo', url }, callback);

// ff2mpv
chrome.runtime.onMessageExternal.addListener((request, _sender, sendResponse) => {
  const { type, url } = request;
  ...
  
  case 'openVideo':
    ff2mpv(url);
    return sendResponse('ok');
  ...
});

I believe that the listener could be included before merging the mv3 migration (so this works with the current version).

Update: It requires to use chrome.runtime.onMessageExternal instead of chrome.runtime.onMessage.

[DanSM-5]

I've opened a PR with the changes for this to work #113

[woodruffw]

Thank you!

[woodruffw]

Addressed by #113. Please file a new issue after that's released if you're still seeing problems here.