Fix missing shifting of Match._textxx fields (#72728)

Fix missing shifting of Match._textxx fields (#72728)

This regressed during the work to support spans.  The Match._textxx fields were set based on the span positions rather than the input string positions, so if the input string had a non-zero beginning value, these fields would be off by that amount.  That then impacts NextMatch, as these values are fed into it.

While fixing this, I noticed that the _textstart field was wholely unnecessary and deleted it.  I also noticed we were unnecessarily passing around some values that weren't needed (e.g. a runtextbeg value that would always be 0), and re-storing the _regex object that can't ever change, and addressed those.