Improve/fix latex math extension. To mitigate false positives: * We accept $ and $$ as a potential opener only if it's not preceded with alnum char. * Similarly closer cannot be followed with alnum char. * We now also match closer with last preceding pontential opener, not the first one. (And to avoid nesting, any previous openers are ignored.) * Also revert an unintended change in 3fc207affaba313cc1f4ef3b4e9e57df89b0e028 which allowed keeping nested resolved marks in it.