CLI option to run arbitrary command

[kanashimia]

#8521 added +N number handling to helix, but in vim +N actually executes command N.
Helix already handles :N so it would be possible to generalise the behaviour.

Possible flag names are: --cmd -C -x -e --run --exec
Possible description:

    -C, --cmd <command>            Specifies a command to run at startup, can be passed multiple times

This flag will be useful for scripting. Another one could be added later to source commands from

There are two ways it could work:

1. It could either allow only named commands, as in helix-term/src/commands.rs static_commands!
2. It could could work like a key sequence input.

Exiting immediately using these two approaches:

1. hx --cmd quit
2. hx --cmd ':quit<ret>'

It is also possible to keep + for the first approach and and add --cmd for the second one.

---

I tried to implement the second approach like this:

for command in args.commands {
    for key_event in helix_view::input::parse_macro(&command).unwrap() {
        app.compositor.handle_event(
            &Event::Key(key_event),
            &mut cx
        );
    }
}

That code is in Application::new just after let app =, but it seems to break on :quit<ret>

› cargo run -- -C ':quit<ret>'  
thread 'main' panicked at helix-view/src/tree.rs:278:29:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I believe it requires view or something to be already created, couldn't figure out how to do that without adding a new event type, as that seemed too complicated for no reason, so for now will just keep this issue here.
I also didn't find an API to implement the first approach.

[the-mikedavis]

This was discussed a little in the initial PR that lead to #8521 #5603 (comment) and I believe elsewhere but I can't find the link. I'm very skeptical that we need this feature. Helix is meant to be run interactively so we try to keep the argument parsing and behavior very simple.