PLT-6706: add /code command (#6936)

* add /code command

* return an error for empty /code message

app/command_code_test.go

@@ -0,0 +1,26 @@
+package app
+
+import (
+	"testing"
+
+	"github.com/mattermost/platform/model"
+)
+
+func TestCodeProviderDoCommand(t *testing.T) {
+	cp := CodeProvider{}
+	args := &model.CommandArgs{
+		T: func(s string, args ...interface{}) string { return s },
+	}
+
+	for msg, expected := range map[string]string{
+		"":           "api.command_code.message.app_error",
+		"foo":        "    foo",
+		"foo\nbar":   "    foo\n    bar",
+		"foo\nbar\n": "    foo\n    bar\n    ",
+	} {
+		actual := cp.DoCommand(args, msg).Text
+		if actual != expected {
+			t.Errorf("expected `%v`, got `%v`", expected, actual)
+		}
+	}
+}