Is your feature request related to a problem? Please describe it.
#1968 discusses conceal support

Describe the solution you'd like
I would like to be able to extend the conceal support to custom commands/macros that I define. As mentioned in the other issue, suppose I have these macros

\newcommand{\mat}[1]{{\mathbf{{#1}}}}
\newcommand{\vect}[1]{{\bm{#1}}}

Then in the text I might write

$ \mat{A} \vect{x} = \vect{b}$

The advantage of this system is that I can easily change the text styles for vectors and matrices. It'd be cool if conceal could work for the \mat and \vect commands, somehow, so that they conceal as A x = b

Describe alternatives you've considered
@lervag suggested allowing for options like

let g:vimtex_syntax_custom_cmds = {
      \ 'bold': ['mat', 'vect'],
      \ 'italic': ['todo'],
      \ 'hide': ['footnote'],
      \}

That would be great. I do not think something more dynamic (that automatically picks up the macro definitions) is needed.