getchar: implement <Cmd> key to invoke command in any mode

src/nvim/keymap.c

@@ -285,6 +285,7 @@ static const struct key_name_entry {
   { K_SNR,             "SNR" },
   { K_PLUG,            "Plug" },
   { K_PASTE,           "Paste" },
+  { K_COMMAND,         "Cmd" },
   { 0,                 NULL }
   // NOTE: When adding a long name update MAX_KEY_NAME_LEN.
 };