feat(messages): add :!cmd shell message kinds

Also print stderr error messages with ErrorMsg highlight group.

test/functional/ui/messages_spec.lua

@@ -331,6 +331,7 @@ describe('ui/ext_messages', function()
       },
     })
 
+    -- kind=verbose for :verbose messages
     feed(':1verbose filter Diff[AC] hi<CR>')
     screen:expect({
       cmdline = { {
@@ -380,6 +381,43 @@ describe('ui/ext_messages', function()
         },
       },
     })
+
+    -- kind=shell for :!cmd messages
+    local cmd = t.is_os('win') and 'echo stdout& echo stderr>&2& exit 3'
+      or '{ echo stdout; echo stderr >&2; exit 3; }'
+    feed(('<CR>:!%s<CR>'):format(cmd))
+    screen:expect({
+      cmdline = { {
+        abort = false,
+      } },
+      messages = {
+        {
+          content = { { (':!%s\r\n[No write since last change]\n'):format(cmd) } },
+          history = false,
+          kind = '',
+        },
+        {
+          content = { { ('stdout%s\n'):format(t.is_os('win') and '\r' or '') } },
+          history = false,
+          kind = 'shell_out',
+        },
+        {
+          content = { { ('stderr%s\n'):format(t.is_os('win') and '\r' or ''), 9, 6 } },
+          history = false,
+          kind = 'shell_err',
+        },
+        {
+          content = { { '\nshell returned 3\n\n' } },
+          history = false,
+          kind = 'shell_ret',
+        },
+        {
+          content = { { 'Press ENTER or type command to continue', 6, 18 } },
+          history = false,
+          kind = 'return_prompt',
+        },
+      },
+    })
   end)
 
   it(':echoerr', function()