Feature: Add support for "Pull Request" as a supported first class command

Currently there is a very limited support around visualizing/modifying the linked pull request.
The only way I could find was to do a `gs ls` and look at the number after the hash. But even this is not the full URL so it's little tricky to open the PR from the terminal itself.
```
my-branch (#135328) ◀
```

I think it would be good to have first class support for pull requests. A few actions I think can think of:
- See the currently linked PR with the full URL so that people can just command click to open it
- Unlink the PR (helpful when we close the PR and would like to open a new one)
- Link to a new PR (helpful when we close and reopen another PR outside of git spice)

PS: Thanks for building this, this has saved me so many hours. I use it everyday! ❤