Highlight feature for elements other than code

[ngaurav]

Is your feature request related to a problem? Please describe. Yes I'm trying to use slidev for step by step solutions to math problems. I thought I could highlight steps like I could highlight code.

Describe the solution you'd like The curly braces syntax for highlight row numbers should be allowed for Katex blocks. E.g. $$ {2-3|all} \begin{array}{c} \nabla \cdot \vec{\mathbf{B}} & = 0 \end{array} $$

Describe alternatives you've considered None