Fix ^ to not match after a string terminating newline

[slevithan]

See shikijs/shiki#866. Oniguruma's ^ doesn't match after a string-terminating \n (though it matches after every other \n, including the position sandwiched between \n\n). There is no equivalent restriction on $.

In other words, oniguruma-to-es needs to change its conversion for ^ from (?<=^|\n) to (?<=^|\n(?!$)).