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_slash_command/src/assistant_slash_command.rs

@@ -6,6 +6,7 @@ use std::sync::Arc;
 use anyhow::Result;
 use futures::channel::oneshot;
 use gpui::{AppContext, Task};
+use language::LspAdapterDelegate;
 
 pub use slash_command_registry::*;
 
@@ -23,7 +24,17 @@ pub trait SlashCommand: 'static + Send + Sync {
         cx: &mut AppContext,
     ) -> Task<Result<Vec<String>>>;
     fn requires_argument(&self) -> bool;
-    fn run(&self, argument: Option<&str>, cx: &mut AppContext) -> SlashCommandInvocation;
+    fn run(
+        self: Arc<Self>,
+        argument: Option<&str>,
+        // TODO: We're just using the `LspAdapterDelegate` here because that is
+        // what the extension API is already expecting.
+        //
+        // It may be that `LspAdapterDelegate` needs a more general name, or
+        // perhaps another kind of delegate is needed here.
+        delegate: Arc<dyn LspAdapterDelegate>,
+        cx: &mut AppContext,
+    ) -> SlashCommandInvocation;
 }
 
 pub struct SlashCommandInvocation {