Katex highlighting and error display

As much of tex code consist of long lines, I think it would be a nice feature to highlight tex functions.

Current behaviour

Expected behaviour

Rather than displaying an error message and stating where the error is it would be more intuitive to display parts of the code that doesn't have an error while displaying the error in another colour like typora.

Current behaviour

Expected behaviour

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[mikaoelitiana]

this is linked to katex support on codemirror cben/CodeMirror-MathJax#39

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