Add post-merge CI to swiftly

[cmcgee1024]

Currently, CI only runs pull requests when they are opened, changed, or someone asks using a comment on a PR. There should be a post-merge CI for swiftly that checks that the merged result on the main branch too so that any merge issues are found right away.

[adam-fowler]

This would be easier if we use GH actions to do our CI for Swiftly

[cmcgee1024]

See #185