No way to remove Milestone

[hbons]

Forgot to add a way to remove a milestone. :)

We could add an item to the quick pick: "Clear Milestone" followed by a separator when a milestone is already set.
Also, we don't currently focus the picked Milestone.

[alexr00]

To verify:

• Install the pre-release version of the extension
• Open a folder that contains a GitHub repo
• Run the "Create Pull Request" command (you won't actually need to create a PR, just run the command).
• Add a milestone to the PR that you're creating
• Verify that you can remove the milestone.

[hbons]

Verified I can remove the milestone after adding.