SetLines sets the line offsets for a file and reports whether it succeeded. The line offsets are the offsets of the first character of each line; for instance for the content "ab\nc\n" the line offsets are {0, 3}. An empty file has an empty line offset table. Each line offset must be larger than the offset for the previous line and smaller than the file size; otherwise SetLines fails and returns false. Callers must not mutate the provided slice after SetLines returns.