Add math environment empheq from package empheq

[kiryph]

Currently, the environment empheq is not considered a math environment:

\begin{empheq}{align*}
a&=b \tag{*}\\
E&=mc^2 + \int_a^a x\, dx
\end{empheq}

The package is hosted on github in the repository https://github.com/latex3/mathtools

Affected parts of vimtex are concealment, the options

• g:vimtex_syntax_custom_cmds with the parameter mathmode
• g:vimtex_syntax_custom_cmds_with_concealed_delims with the parameter mathmode

and the helper function

• vimtex#syntax#in_mathzone e.g. used by https://github.com/iurimateus/luasnip-latex-snippets.nvim

[kiryph]

I have added myself:

❯ cat autoload/vimtex/syntax/p/empheq.vim

" VimTeX - LaTeX plugin for Vim
"
" Maintainer: Karl Yngve Lervåg
" Email:      karl.yngve@gmail.com
"

scriptencoding utf-8

function! vimtex#syntax#p#empheq#load(cfg) abort " {{{1
  call vimtex#syntax#core#new_region_math('empheq')
endfunction

[lervag]

I see no reason to not just add this...