Automate release process.

[polina-c]

Working notes: go/a2ui-release-notes
Outcome: go/a2ui-release

• Pub.dev - polina - after every pull request
  • Research
  • Permissions: Configure repository to allow auto-release. #1429
  • Configure
  • Test
  • Document
• Npm - david
• Google3 - david
• Pypi - David will ask nan
• Documentation - automation done, but not documented - polina will ask Alan

[ditman]

Removing @selamw1, since this is a process-only thing!