Support checking out a PR through a URI handler

We should be able to support the following URI:

`vscode://github.vscode-pull-request-github?action=checkout&url=https%3A%2F%2Fgithub.com%2FMicrosoft%2Fvscode%2Fpull%2F61444`

This should result in a state that is the same as when I manually navigate to `https://github.com/Microsoft/vscode/pull/61444`, open the Description page, and checkout the PR.
