feat(fold): support comment package

refer: #2882

autoload/vimtex/fold.vim

@@ -55,6 +55,7 @@ function! vimtex#fold#init_state(state) abort " {{{1
         \ . '|^\s*}'
   let a:state.fold_re_next = ''
   for l:name in [
+        \ 'comment_pkg',
         \ 'preamble',
         \ 'cmd_single',
         \ 'cmd_single_opt',

autoload/vimtex/fold/comment_pkg.vim

@@ -0,0 +1,39 @@
+" VimTeX - LaTeX plugin for Vim
+"
+" Maintainer: Karl Yngve Lervåg
+" Email:      karl.yngve@gmail.com
+"
+
+function! vimtex#fold#comment_pkg#new(config) abort " {{{1
+  return extend(deepcopy(s:folder), a:config)
+endfunction
+
+" }}}1
+
+
+let s:folder = {
+      \ 'name' : 'comment_pkg',
+      \ 're' : {
+      \   'start' : '^\s*\\begin\s*{comment}',
+      \   'end' : '^\s*\\end\s*{comment}',
+      \ },
+      \ 'opened' : v:false,
+      \}
+function! s:folder.level(line, lnum) abort dict " {{{1
+  if a:line =~# self.re.start
+    let self.opened = v:true
+    return 'a1'
+  elseif a:line =~# self.re.end
+    let self.opened = v:false
+    return 's1'
+  elseif self.opened
+    return '='
+  endif
+endfunction
+
+" }}}1
+function! s:folder.text(line, level) abort dict " {{{1
+  return a:line
+endfunction
+
+" }}}1

autoload/vimtex/options.vim

@@ -95,6 +95,7 @@ function! vimtex#options#init() abort " {{{1
   call s:init_option('vimtex_fold_types_defaults', {
         \ 'preamble' : {},
         \ 'items' : {},
+        \ 'comment_pkg' : {},
         \ 'comments' : { 'enabled' : 0 },
         \ 'envs' : {
         \   'blacklist' : [],

doc/vimtex.txt

@@ -1846,6 +1846,9 @@ OPTIONS                                                        *vimtex-options*
                         corresponding "real" sections. The fold title is the
                         provided title with the `Fake...` part prepended.
 
+    <comment_pkg>       Fold `\begin{comments} ... \end{comments}` and disable
+                        folding inside the environment.
+
     <comments>          Fold multiline comments. This is disabled by default.
 
     <markers>           Fold on vim-style markers inside comments, that is,
@@ -1928,6 +1931,7 @@ OPTIONS                                                        *vimtex-options*
     let g:vimtex_fold_types_defaults = {
           \ 'preamble' : {},
           \ 'items' : {},
+          \ 'comment_pkg' : {},
           \ 'comments' : {'enabled' : 0},
           \ 'envs' : {
           \   'blacklist' : [],