Speed up tab searching in `TabMap::sync` by looking for `\t` only

Instead of looking for `\n` as a stopping condition, we cap the range
we pass to `SuggestionSnapshot::chunks` to stop on the next line. This
makes character searching faster, because looking for a single character
uses `memchr`. Also, this avoids an extra conditional in a tight loop such
as the chunk scanning one contained in `TabMap::sync`.