feat(messages): hl-StderrMsg, hl-StdoutMsg #33429

Problem:
stderr messages from executing ":!cmd" show up with
highlight hl-ErrorMsg. But some shell utilites use stderr for debug
logging, progress updates, etc.

Solution:
Highlight shell command outputs hl-StderrMsg and hl-StdoutMsg.

src/nvim/highlight_group.c

@@ -180,6 +180,8 @@ static const char *highlight_init_both[] = {
   "default link Substitute       Search",
   "default link StatusLineTerm   StatusLine",
   "default link StatusLineTermNC StatusLineNC",
+  "default link StderrMsg        ErrorMsg",
+  "default link StdoutMsg        NONE",
   "default link TabLine          StatusLineNC",
   "default link TabLineFill      TabLine",
   "default link VertSplit        WinSeparator",