Avoid matching `\iff` in syntax highlighting comment matching

[user202729]

Basically currently if there's some \iffalse ... \fi it will highlight the block as a comment, but \iffalse $ a \iff b $ \fi more content will also highlight more content as comment because it thought \iff needs to be matched.

Remark: generally TeX is not deterministically parsable anyway, but \iff is a very common case. Looking around, some less common ones are \ifcurrfiledir etc. (currfile package) or ifnot@nil (macros2e) .

[lervag]

Agh, yes, TeX is hard to parse! But in this case, I believe it should not be too hard - we just need to recognize that \iff does not start a new \if ... \fi group.

[lervag]

I believe this is fixed now.