Add support for n-th roots, e.g. `\sqrt[3]{2}`?

[fingolfin]

This doesn't seem to be supported right now?

julia> texparse(raw"\sqrt[3]{2}")
ERROR: TeXParseError: end of string reached with unfinished argument_gatherer
at position 10 (string index 10)
\sqrt[3]{2}
         ^
Stack before
...

[Kolaru]

It is indeed not supported.

It is on my list, but since it requires the support for optional argument in commands, I will need to add some rules to the parser and it is always a bit tricky.