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/slash_command/file_command.rs

@@ -3,6 +3,7 @@ use anyhow::Result;
 use futures::channel::oneshot;
 use fuzzy::PathMatch;
 use gpui::{AppContext, Model, Task};
+use language::LspAdapterDelegate;
 use project::{PathMatchCandidateSet, Project};
 use std::{
     path::Path,
@@ -96,7 +97,12 @@ impl SlashCommand for FileSlashCommand {
         })
     }
 
-    fn run(&self, argument: Option<&str>, cx: &mut AppContext) -> SlashCommandInvocation {
+    fn run(
+        self: Arc<Self>,
+        argument: Option<&str>,
+        _delegate: Arc<dyn LspAdapterDelegate>,
+        cx: &mut AppContext,
+    ) -> SlashCommandInvocation {
         let project = self.project.read(cx);
         let Some(argument) = argument else {
             return SlashCommandInvocation {