feat(math): Support square roots

[Omikhleia]

Closes #1604

My dear math friends... Do you really expect SILE will thrive without the square root? I don't know for other countries, but in France, at least when I was younger, the solution to second-degree equations was taught in school much earlier than integrals, matrices and other advanced stuff.

So we shall all want square roots, even if it has to be a hack of some sort.

Not supporting these is detrimental to SILE, when other worse solutions are proposed to typists (pun intended and completely assumed)

As I just said...

[Omikhleia]

Note: I would have wished for the "inline" (text mode) last formula to make the a_n smaller and smaller. Would you help, oh my great math friends?

EDIT: LaTeX (below) does not seem to make roots in roots smaller, actually.

We are all good then. Well LaTeX shows the radical also extending below the baseline (even more so for display math), but there doesn't seem to be an openType property corresponding to this feature... (EDIT: And using for reference the depth the radical symbol would possibly have in the current font doesn't lead to such huge expansion - So I am unsure we want to care.)

[Omikhleia]

Be my guests:

\begin[class=book, papersize=a6]{document}
\use[module=packages.math]
\font[family=Libertinus Serif]

We want square roots...
\math[mode=display, numbered=true]{\sqrt{-1} = i}

Display:
\math[mode=display, numbered=true]{x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}}

Or inline: \math{x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}}

\math[mode=display, numbered=true]{c = \sqrt{\frac{E}{m}}}

Or inline: \math{c = \sqrt{\frac{E}{m}}}

Let's go mad, for any sequence \math{a_n > 0}:
\math[mode=display, numbered=true]{\sqrt{a_1 + \sqrt{a_2 + \sqrt{a_3 + \sqrt{a_4 + \sqrt{a_5 ...}}}}}
 = e^{\sum_{n=1}^{\infty} \frac{\mi{ln}\, a_n}{2^n}}
}

Inline: \math{\sqrt{a_1 + \sqrt{a_2 + \sqrt{a_3 + \sqrt{a_4 + \sqrt{a_5 ...}}}}}
 = e^{\sum_{n=1}^{\infty} \frac{\mi{ln}\, a_n}{2^n}}}

\end{document}