refactor(complete): Add an alias for the subcommand

I want to add support for completion of arguments for aliased
subcommands so it's nice to have an example to test on.

clap_complete/examples/completion-derive.rs

@@ -30,6 +30,7 @@ struct Opt {
 
 #[derive(Subcommand, Debug, PartialEq)]
 enum Commands {
+    #[clap(visible_alias = "hint")]
     ValueHint(ValueHintOpt),
 }
 

clap_complete/examples/completion.rs

@@ -18,6 +18,7 @@ use std::io;
 
 fn build_cli() -> Command {
     let value_hint_command = Command::new("value-hint")
+        .visible_alias("hint")
         .arg(
             Arg::new("unknown")
                 .long("unknown")