vsce: implement new auth flow ux

Created by: abeatrix

Close https://github.com/sourcegraph/sourcegraph/issues/35899

ON HOLD - current waiting on Sara for the design of the redirect page

move the new callback page to the existing new token creation page to provide backward compatibility (kind of), so that if users are on older version, they will now be redirected to a 404 page

Test plan

App preview:

• Web

Check out the client app preview documentation to learn more.