Support TeX-like \sqrt[3]{...} for roots with degrees

[Omikhleia]

( Split from #2120 under its own topic, with a clearer title )
Since the latter, and SILE 0.15.6, we support MathML's \mroot.

In TeX-like math, we do not support \sqrt[...]{....} however.

This would require a non-trivial but interesting extension of our TeX-like math parser.

Obviously a workaround is to use \mroot{...}{....} for now (since we can mix TeX-like math and MathML).

As shown, this does work, but under the principle of least astonishment, the ideal would be to support as much as possible a decent subset of the TeX math syntax, hence this issue.

Additional reference for the TeX-math case:

https://github.com/jgm/texmath/blob/master/src/Text/TeXMath/Readers/TeX.hs#L687-L691