Is your feature request related to a problem? Please describe.
It seems that optional arguments are not supportes by user defined \newcommand.
Just see the following code.

\newcommand \foo [2] [bar] {{#1}^{#2}}
\foo {1}
\foo [baz]{2}

Describe the solution you'd like:
Would this optional argumant feature be implemented just like \sqrtmacro? this is a predefined macro in KaTeX with optional argument support.

Link to or name of a (La)TeX package that provides the same feature:
N/A

Describe alternatives you've considered:
N/A

Additional context:
N/A