Add option to checkout PR on Codespaces

[luabud]

Testing #7424

It'd be nice if I had an option to checkout the PR on Codespaces. I was working on a local repo that wasn't set up on my machine, and I wish I had the option from the PR header to checkout on Codespaces so that the repo would be set up for me to test my changes

[alexr00]

To verify:

• Install the latest pre-release version of GitHub Pull Requests
• Open a folder which contains a GitHub repo
• From the Pull Requests view, open a PR description
• Use the dropdown button to "Checkout on Codespaces"