Auto-update: Mac

[timtmok]

Mac can use Electron to download and update Positron.

• Add version check
• Provide an action to restart to install update

[timtmok]

I enabled the auto-update on my Positron client and tried to update with the staging channel. It worked! The client was able to download and notify that a restart was needed to complete the update. Upon restarting, the client was at the new version.

There is still work to be done on the publishing side. Mac requires that the update to be a zip containing Positron.app. This is the only format that I'm aware of that will work. Electron will handle unzipping and checking the app bundle id before installing the update.