Add support for `\vspace` command

Is your feature request related to a problem? Please describe.

MathJax currently lacks support for the \vspace command. This makes it difficult to adjust vertical spacing in equations, particularly for negative spacing. While \hspace is supported, there's no direct equivalent for vertical space.

For example, consider trying to create a diagram, such as:

\begin{align}
    \hspace{4cm}      Y                 \\
    \hspace{4cm}      \rule{0.1em}{2cm} \\
    % \vspace{-1em}
    X  \hspace{0.2em} \rule{3.5cm}{0.1em}
\end{align}

$$
\begin{align}
\hspace{4cm} Y \
\hspace{4cm} \rule{0.1em}{2cm} \
% \vspace{-1em}
X \hspace{0.2em} \rule{3.7cm}{0.1em}
\end{align}
$$

Without \vspace, it is impossible to introduce the desired negative vertical spacing.

Describe the solution you'd like

Add support for the \vspace command in MathJax, allowing users to add vertical space (both positive and negative) using syntax like \vspace{1em} or \vspace{-1em}.

Describe alternatives you've considered

The workaround \rule{0pt}{#1em} can add positive vertical space but cannot create negative vertical space.

No source file, test, or entry point is named in the issue. Start by tracing the existing \hspace command in MathJax and compare its behavior with the requested \vspace{1em} and \vspace{-1em} cases; done means both positive and negative vertical spacing work in the shown align example.