chore(cli): rename Cmd to Command (#12616)

I think Command is cleaner and my original decision to use "Cmd"
a mistake.

Plus this creates better parity with cobra.

cli/exp.go

@@ -2,15 +2,15 @@ package cli
 
 import "github.com/coder/serpent"
 
-func (r *RootCmd) expCmd() *serpent.Cmd {
-	cmd := &serpent.Cmd{
+func (r *RootCmd) expCmd() *serpent.Command {
+	cmd := &serpent.Command{
 		Use:   "exp",
 		Short: "Internal commands for testing and experimentation. These are prone to breaking changes with no notice.",
 		Handler: func(i *serpent.Invocation) error {
 			return i.Command.HelpHandler(i)
 		},
 		Hidden: true,
-		Children: []*serpent.Cmd{
+		Children: []*serpent.Command{
 			r.scaletestCmd(),
 			r.errorExample(),
 		},