feat: Display math equation numbers now use a resilient style (eqno)

[Omikhleia]

... and also subscribe for labelrefs cross-referencing.

Closes #90

A SIL test input file:

\begin[class=resilient.book, papersize=a6]{document}
\font[family=Libertinus Serif]
\math[mode=display, numbered=true]{e^{i\pi} = -1}
\math[mode=display, number=2A]{e^{i\pi} = -1}
\math[mode=display, counter=stuff]{e^{i\pi} = -1}
\math[mode=display, marker=myeq, counter=stuff]{e^{i\pi} = -1}
\math[mode=display, counter=stuff]{e^{i\pi} = -1}

Equation \ref[marker=myeq] is especially remarkable.
\end{document}

Let's use a totally dumb style (purple color, oldstyle numbers, roman numbering, square brackets instead of parentheses), because... why not.

eqno:
  style:
    font:
      features: "+onum"
    color: "purple"
    numbering:
      after:
        text: "]"
      before:
        text: "["
      display: "roman"

Yay!

Solid and consistent 🤣

[Omikhleia]

Wait. Let's go full style-enabled and add something for our custom counter:

eqno-stuff:
  inherit: "eqno"
  style:
    color: "red"
    numbering:
      display: "alpha"

Oh yay, roll baby.