VSCE: Update event name and eventSource type for usage metrics collection
Created by: abeatrix
This PR includes:
- Updated event name
VSCESearchSubmitted
toIDESearchSubmitted
- Use new eventSource type
EventSource.IDEEXTENSION
when connected to a Sourcegraph instance that's running version 3.38.0 or above - Continue to use eventSource type
EventSource.BACKEND
when connected to an older Sourcegraph instance