Handle mtable (`\begin{equation}`, align, gather, matrix, …) in KaTeX

[rajveermalviya]

A LaTeX environment such as \begin{equation}/\end{equation} (or equation*, gather, align, matrix, cases, or any of the other environments KaTeX supports) causes KaTeX to emit spans with .mtable and one or more of its corresponding .vertical-separator, .arraycolsep, .col-align-c, .col-align-l, .col-align-r child spans. See the .mtable rules in katex.scss.

This was 16th item in our survey (#mobile-team > KaTeX survey results @ 💬), which covered one community; but we heard a user request for it: #mobile > LaTeX with accents not rendering @ 💬.

[gnprice]

Quoting that bit of katex.scss here for convenience:

    .mtable {
        .vertical-separator {
            display: inline-block;
            // margin and border-right are set in JavaScript
            min-width: 1px;  // Prevent Chrome from omitting a line.
        }

        .arraycolsep {
            display: inline-block;
        }

        .col-align-c > .vlist-t {
            text-align: center;
        }

        .col-align-l > .vlist-t {
            text-align: left;
        }

        .col-align-r > .vlist-t {
            text-align: right;
        }
    }

[gnprice]

So this is about alignment in vlist structures. Rajesh tells me this will therefore probably be straightforward after that alignment is implemented, which will also be needed for #1674 and #1675.