Add command and shortcut to copy PR link

[albireox]

In the PR page it's possible to copy the link to the PR (something like https://github.com/org/repo/pull/14). But as far as I can see there is no command to do that from the command palette and it's not possible to assign it a shortcut. This would be very useful when writing change logs. Ideally it would also be possible to copy the link in Markdown, for example [#14](https://github.com/orgrepo/pull/14).