Patch `replaceMathString` to not render math in codes

[TroyDanielFZ]

A regex expression to exclude the code tag with the help of look-around operation is not preferred, due to: This is a $x$ to produce $x$, and a $y$ to produce $y$, If we apply the look-around, the first capture will be '$ to produce $', which is not correct Therefore, a simple way, split the src at 'code block', Known bugs: "`$abcd$`" and "\$abcd$\" won't render, actually, I don't hnow what's the espected result.

[volca]

Thanks. I will make a little code format.