This fixes an issue where capture groups inside a positive lookahead
were being reset even upon successful matching of the lookahead. For
example, with the pattern `/(?=(\d))/`, matching against a string like
`"abc1"` should result in the output `("", "1")`. However, accessing
the output traps instead, since the range data for capture 1 is
missing even on success.

This change resolves the issue by adding a boolean payload to the
`fail` instruction that indicates whether to preserve captures when
resetting the matching state, which allows any captures inside a
lookahead to persist after success.

Fixes #713.