First party implementation of release fetching and unpacking

[anfragment]

Self-update algorithm

1. Fetch the release manifest
2. Check if manifest version > current version
3. Download the file from assetURL into a temporary directory
4. Check if the file hash matches that of the release
5. Replace:
   • On macOS, the entire .app bundle
   • On Windows and Linux, just the currently running binary

References

• https://github.com/rhysd/go-github-selfupdate/tree/master/selfupdate
• https://github.com/anfragment/zen/blob/master/internal/autostart/autostart.go#L13