regex engine: Issue #19168 - Fix variable length lookbehind matches

regex engine: Issue #19168 - Fix variable length lookbehind matches

We were not validating that when (?<=a|ab) matched that the right hand
side of the match lined up with the position of the assertion. Similar
for (?<!a|ab) and related patterns, eg, (*positive_lookbehind:).
Note these problems do NOT affect lookahead.

Part of the difficulty here was that the SUCCEED node was serving too
many purposes, necessitating a new regop LOOKBEHIND_END.

Includes more tests for various lookahead or lookbehind cases.