Skip TeX comments correctly. mathjax/MathJax#2271.

[dpvc]

Update FindTeX to properly skip comments, even if they include unbalanced braces.

Resolves issue mathjax/MathJax#2271

[zorkow]

lgtm.

[dpvc]

This is potentially a breaking change, so I am not going to include it in this version. The problem is that

... $$ x + y % comment $$ ...

would be processed by the current version (and version 2), but would not be processed by this PR, since the second $$ is now part of a comment. Although the behavior of this PR is the correct one for LaTeX, it might break existing content that takes advantage of this error in processing.

[dpvc]

There should also be additional changes to actually remove all comments before processing the rest of the expression, as currently that is not being done. For example,

\def\xyz#1%#2{[#1][#2]}

is valid, and can be used via \xyz a%b to get [a][b] as output. This is not how actual LaTeX works, and so this should be fixed as well. But I know that this is used on the Puzzling StackExchange site in some MathJax-based puzzles, and it will break those.

So I'm wondering if it is better to leave it as is, or break those incorrect (but useful) usages?