feat: Add o as a command to open up a pr on github

[basola21]

Hey, I have been having fun with jj and your work here is amazing, anyhow recently I have added some of my changes, I do not know if you may like it (it is a little opinionated) , anyhow it is just another command to use o as a pr opener on github, nothing crazy there

• please ignore the previous two commits, I was just playing around, if you want this as a feature I could clean it up to use config keys -> better for users that are not using github.

thanks for this amazing tool

[peso]

I like the idea of getting more help to do frequent stuff. I think you should split this into two PR: One for AI assistance and one for Github. Also, it should be more configurable - don't hardcode the URL to your AI.