Add ChunkMatch.BestLineMatch to return the best-scoring line (#884)
This PR adds a new field `ChunkMatch.BestLineMatch` with the line number of top-scoring line in the chunk. This will let us address a long-standing issue with our new flexible keyword search, where chunk matches can become very large. Since our search results UX only shows the start of a chunk, the most relevant line may not even be visible. With this information on the best line match, we can adjust the search results UX to center the chunk on the most relevant line.
Relates to [SPLF-188](https://linear.app/sourcegraph/issue/SPLF-188/ensure-the-best-scoring-line-match-is-shown-in-search-results)