Performance: Prefetch file blobs from search results

Created by: umpox

Description

This PR adds support for prefetching blobs from the search results through hover or focus, through the experimental feature enableSearchFilePrefetch

This should significantly speed up time from search results -> viewing code, especially now we have no waterfall logic between these components

Before

https://user-images.githubusercontent.com/9516420/188905099-6e013ddd-ea29-4018-8877-eb21844ce08a.mp4

After

https://user-images.githubusercontent.com/9516420/188905204-8ae0b1d4-6876-442c-b9ed-a5f91c9b2431.mp4

Test plan

Tested locally with different configurations of highlighting / file viewer