Allow defining slash commands in extensions (#12255)

This PR adds initial support for defining slash commands for the
Assistant from extensions.

Slash commands are defined in an extension's `extension.toml`:

```toml
[slash_commands.gleam-project]
description = "Returns information about the current Gleam project."
requires_argument = false
```

and then executed via the `run_slash_command` method on the `Extension`
trait:

```rs
impl Extension for GleamExtension {
    // ...

    fn run_slash_command(
        &self,
        command: SlashCommand,
        _argument: Option<String>,
        worktree: &zed::Worktree,
    ) -> Result<Option<String>, String> {
        match command.name.as_str() {
            "gleam-project" => Ok(Some("Yayyy".to_string())),
            command => Err(format!("unknown slash command: \"{command}\"")),
        }
    }
}
```

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -237,6 +237,7 @@ pub fn init(client: Arc<Client>, cx: &mut AppContext) {
     cx.set_global(Assistant::default());
     AssistantSettings::register(cx);
     completion_provider::init(client, cx);
+    assistant_slash_command::init(cx);
     assistant_panel::init(cx);
 
     CommandPaletteFilter::update_global(cx, |filter, _cx| {