Search backend: add chunk matches to the searcher protocol
Created by: camdencheek
This adds adds the ChunkMatch type as an optional return type for the searcher API. This means there are now three different result types returnable by the searcher API, but both LineMatch and MultilineMatch are superceded by ChunkMatch. The next steps will be to remove LineMatch and MultilineMatch from the API.
Test plan
Not used yet. Will be exercised when searcher is migrated to return ChunkMatch
instead of LineMatch
and MultilineMatch
.