cli: move jj co alias to config file, so it can be overridden

This way users can override `jj co` to mean `jj new` if they want to
get rid of the warning.

cli/src/commands/checkout.rs

@@ -25,7 +25,6 @@ use crate::ui::Ui;
 /// For more information, see
 /// https://github.com/martinvonz/jj/blob/main/docs/working-copy.md.
 #[derive(clap::Args, Clone, Debug)]
-#[command(visible_aliases = &["co"])]
 pub(crate) struct CheckoutArgs {
     /// The revision to update to
     revision: RevisionArg,

cli/src/config/misc.toml

@@ -1,5 +1,7 @@
+# The code assumes that this table exists, so don't delete it even if you remove
+# all aliases from here.
 [aliases]
-# Placeholder: added by user
+co = ["checkout"]
 
 [format]
 tree-level-conflicts = true