Added $priority function to AFUnwrappedDataSnapshot

[lyonzy]

Checklist

• Issue number for this PR: 1164
• Docs included?: (no)
• Test units included?: (no)
• In a clean directory, npm install, npm run build, and npm test run successfully? (no - doesn't seem to agree with Windows)

Description

Minor change to utils.ts and interfaces.ts to support $priority.

[davideast]

Hi @lyonzy! Thank you for taking the time to send in a PR. I'd love to accept this, but in #1158 we are changing the API. In that change the AFUnwrappedSnapshot is going away. Instead we are providing more options to get the data without having to unwrap. I recommend giving it a read and see what's coming soon. I expect to merge it this week.

I unfortunately will close this PR but I do appreciate the thought and time you put into this!