VSCE: Change onboarding to favour private instances

Created by: ryankscott

We are now announcing that we will no longer be supporting adding personal repositories on Sourcegraph.com. Consequently we should change our VS Code onboarding flow to prioritise adding the instance URL and the access token, instead of assuming you're going to auth against sourcegraph.com

There may be other changes needed, but this is the most obvious I could find.