Fix x'^2

[edemaine]

Fix for #634. Basically, ' should be treated like a superscript constructor, where any number of 's become \primes in the superscript, and an optional following ^... can define the rest of the superscript. As a result, x^2' does not work (double superscript), and nor does x'_2' (double superscript). All error behaviors now correctly match LaTeX. (x'_2' was accepted by KaTeX previously, but that was incorrect.)

[gagern]

Great! Very quick response, nice code, and it even does the same thing as LaTeX for input like $$\sum'^x$$ or $x'^{x'^{x'^{x'}}}$. The failing 1'_2^3 was not something I would have expected before your explanation, but it does make sense now and it is consistent with LaTeX, too. I'll accept this PR.