Add vertical bar inside augmented matrices

[rbeezer]

Motivated by #92, it would be a small project to add "the vertical bar" to each augmented matrix.

This answer, duplicated here, seems workable via a 2-argument macro.

https://tex.stackexchange.com/a/535692

Behaves well at https://www.tuhh.de/MathJax/test/sample-dynamic.html

\left[
  \begin{matrix}
    1 & 2 & 3 \\
    1 & 2 & 3 \\
    1 & 2 & 3 \\
    1 & 2 & 3 \\
  \end{matrix}
  \left|
    \,
    \begin{matrix}
      4  \\
      4  \\
      4  \\
      4  \\
    \end{matrix}
  \right.
\right]

[kcrisman]

That's a clever minimalist website.