Adopt VSCode Authentication Provider API

[RMacfarlane]

Now that VSCode ships with a GitHub authentication provider, we can leverage it to get the access token we need. This will help unify the accounts UI, so we can remove the status bar item.

[IllusionMH]

Looks like after this change I'm unable to access org repos without separate approve from admins. Is it expected change?

[drayfar]

After this change, I can no longer sign in to GitHub Enterprise using a PAT - the command GitHub Pull Requests: Manually Provide Authentication Response listed in the Wiki no longer exists.