SearchBackend: refine HunkMatch to include PreviewStart
Created by: camdencheek
This refines the new HunkMatch type to include PreviewStart so that we have both the line number and the byte offset of Preview available for further processing. PreviewEnd can always be calculated from Preview, so I felt it was best to just provide a start location rather than a range that is represented by Preview (orthogonal information and all).
Stacked on #36124
Test plan
Updated tests, still passed.