MathML mroot and TeX-like root with degree not supported.

[Omikhleia]

For square roots, see #1604 and PR #2119

But now we shall want n-roots, don't we? Cubic roots, etc.

• [x] MathML "mroot" would just be an extension of PR #2119 with a "child" for the degree/index part... = see #2141
• [ ] TeX-like: \sqrt[3]{...} = Ah. Our TeX-like parser does not seem to have provision for such "optional" square-bracketed content...

My dear math friends... Ye all know we shall all want n-roots...

[Omikhleia]

Additional reference for the TeX-math case:

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