Closes https://github.com/zed-industries/zed/issues/38558

The bug occurred because TabStopCursor chunk_position.1 is bounded
between 0 and 128. The fix for this was changing the bound to 0 and 127.

This also allowed me to simplify some of the tab stop cursor code to be
a bit faster (less branches and unbounded shifts).

Release Notes:

- N/A