Allow to easily open a PR or reveal it in the PR viewlet

[alexdima]

When having a PR open in a browser tab, I feel there is no easy way to open the same PR in vscode. I recently discovered the GHPR: Checkout Pull Request command in the Command Pallette but that doesn't seem to do anything. The other one, GHPR: Checkout Pull Request By Number will immediately modify the working state and not open it. Maybe the new tree async search API could help here.

[alexr00]

Discussed this on Wednesday with Joao: I think we need a transient subtree or view in the PRs view for "show pull request by number".