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:
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