Add Cmd::Macro for keystroke replay functionality

[ticpu]

This commit implements a new Cmd::Macro variant that allows replaying sequences of keystrokes character-by-character, enabling function key bindings to execute commands automatically.

Since this is replaying a command, it integrates with the command processing pipeline seamlessly, without allowing for completely arbitrary commands to be execute.

Example usage:

// Bind F1 to execute "help" command automatically
rl.bind_sequence(
    KeyEvent(KeyCode::F(1), Modifiers::NONE),
    Cmd::Macro("help\n".to_string())
);

When triggered, the macro replays each character as if the user typed it, and the trailing newline triggers AcceptLine to submit the command automatically.

It is related to issue #418: Execute Arbitrary Command Via Keybinding, I took insipration from that issue's name as I needed to "execute" arbitrary command (and not executable) in my application.

[gwenn]

See https://github.com/kkawakam/rustyline/blob/master/CustomBinding.md#multiple--complex-actions

[ticpu]

@gwenn Hah, that's interesting, thanks for sharing.

I think both could co-exist in this case. Macro is addressing a couple use-cases by simulating the user's input; so it can't really get "deprecated"; I see the potential of adding the more complete Custom Bindings but right now, the only alternative would be to move to a different library until it is fully implemented.

I had to write this so I could have a feature-complete fs_cli (FreeSWITCH's CLI).

Would you see any issues to be able to do it both ways? It could even use the custom bindings architecture by converting the text to actions later on.