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_defs.h

@@ -130,6 +130,8 @@ typedef enum {
   HLF_BFOOTER,    ///< Float Border Footer
   HLF_TS,         ///< status line for terminal window
   HLF_TSNC,       ///< status line for non-current terminal window
+  HLF_SE,         ///< stderr messages (from shell)
+  HLF_SO,         ///< stdout messages (from shell)
   HLF_COUNT,      ///< MUST be the last one
 } hlf_T;