feat: Add API for external extensions to call ff2mpv

[DanSM-5]

Add support for external extensions

To allow ff2mpv extension listening to messages it needs to implement browser.runtime.onMessageExternal.

The shape of the message is an object with two properties:

• type: The type of message to handle
• url: The url string to be used by ff2mpv

External extensions need to call ff2mpv using the extension id:

• Firefox: 'ff2mpv@yossarian.net'
• Chrome: 'ephjcajbkgplkjmelpglennepbpmdpjg'

Example of use for external extensions:

// Firefox
browser.runtime.sendMessage('ff2mpv@yossarian.net', { type: 'openVideo', url: 'https://someurl.com' }, callback);

// Chrome
chrome.runtime.sendMessage('ephjcajbkgplkjmelpglennepbpmdpjg', { type: 'openVideo', url: 'https://someurl.com' }, callback);

Solves: https://github.com/woodruffw/ff2mpv/issues/112