fix: keep listchars dimmed in Vim

Vim doesn't distinguish between hl-Whitespace and hl-SpecialKey for
'listchars' and non-listchars unprintable characters, so 'listchars'
ended up being highlighted with a bright color in Vim after commit
767b547.

Reverts the highlight of 'listchars' to the style prior to commit
767b547, although this reduces the readability of mappings in the
output of :map commands.

Fixes #211

colors/gruvbox-material.vim

@@ -10,7 +10,7 @@
 let s:configuration = gruvbox_material#get_configuration()
 let s:palette = gruvbox_material#get_palette(s:configuration.background, s:configuration.foreground, s:configuration.colors_override)
 let s:path = expand('<sfile>:p') " the path of this script
-let s:last_modified = 'Mon Jul  8 07:54:35 UTC 2024'
+let s:last_modified = 'Sat Jul 13 15:32:35 UTC 2024'
 let g:gruvbox_material_loaded_file_types = []
 
 if !(exists('g:colors_name') && g:colors_name ==# 'gruvbox-material' && s:configuration.better_performance)
@@ -135,8 +135,12 @@ call gruvbox_material#highlight('ModeMsg', s:palette.fg0, s:palette.none, 'bold'
 call gruvbox_material#highlight('MoreMsg', s:palette.yellow, s:palette.none, 'bold')
 call gruvbox_material#highlight('MatchParen', s:palette.none, s:palette.bg4)
 call gruvbox_material#highlight('NonText', s:palette.bg5, s:palette.none)
-call gruvbox_material#highlight('Whitespace', s:palette.bg5, s:palette.none)
-call gruvbox_material#highlight('SpecialKey', s:palette.orange, s:palette.none)
+if has('nvim')
+  call gruvbox_material#highlight('Whitespace', s:palette.bg5, s:palette.none)
+  call gruvbox_material#highlight('SpecialKey', s:palette.orange, s:palette.none)
+else
+  call gruvbox_material#highlight('SpecialKey', s:palette.bg5, s:palette.none)
+endif
 call gruvbox_material#highlight('Pmenu', s:palette.fg1, s:palette.bg3)
 call gruvbox_material#highlight('PmenuSbar', s:palette.none, s:palette.bg3)
 if s:configuration.menu_selection_background ==# 'grey'