Support darray `Unknown environment 'darray'`

[universemaster]

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

• \begin{darray}{cc} a & b \\ c & d \end{darray} is a display mode array.
• Mathjax gives Unknown environment 'darray'

Describe the solution you'd like Support darray as KaTex does. Initially darray is from the latex breqn package - page 100.

Describe alternatives you've considered I don't want to put my math on github if the markdown preview is full of unknown environment errors.