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/extension/src/extension_store_test.rs

@@ -5,6 +5,7 @@ use crate::{
     ExtensionIndexThemeEntry, ExtensionManifest, ExtensionStore, GrammarManifestEntry,
     RELOAD_DEBOUNCE_DURATION,
 };
+use assistant_slash_command::SlashCommandRegistry;
 use async_compression::futures::bufread::GzipEncoder;
 use collections::BTreeMap;
 use fs::{FakeFs, Fs, RealFs};
@@ -156,6 +157,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
                         .into_iter()
                         .collect(),
                         language_servers: BTreeMap::default(),
+                        slash_commands: BTreeMap::default(),
                     }),
                     dev: false,
                 },
@@ -179,6 +181,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
                         languages: Default::default(),
                         grammars: BTreeMap::default(),
                         language_servers: BTreeMap::default(),
+                        slash_commands: BTreeMap::default(),
                     }),
                     dev: false,
                 },
@@ -250,6 +253,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
 
     let language_registry = Arc::new(LanguageRegistry::test(cx.executor()));
     let theme_registry = Arc::new(ThemeRegistry::new(Box::new(())));
+    let slash_command_registry = SlashCommandRegistry::new();
     let node_runtime = FakeNodeRuntime::new();
 
     let store = cx.new_model(|cx| {
@@ -262,6 +266,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
             node_runtime.clone(),
             language_registry.clone(),
             theme_registry.clone(),
+            slash_command_registry.clone(),
             cx,
         )
     });
@@ -333,6 +338,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
                 languages: Default::default(),
                 grammars: BTreeMap::default(),
                 language_servers: BTreeMap::default(),
+                slash_commands: BTreeMap::default(),
             }),
             dev: false,
         },
@@ -382,6 +388,7 @@ async fn test_extension_store(cx: &mut TestAppContext) {
             node_runtime.clone(),
             language_registry.clone(),
             theme_registry.clone(),
+            slash_command_registry,
             cx,
         )
     });
@@ -460,6 +467,7 @@ async fn test_extension_store_with_gleam_extension(cx: &mut TestAppContext) {
 
     let language_registry = project.read_with(cx, |project, _cx| project.languages().clone());
     let theme_registry = Arc::new(ThemeRegistry::new(Box::new(())));
+    let slash_command_registry = SlashCommandRegistry::new();
     let node_runtime = FakeNodeRuntime::new();
 
     let mut status_updates = language_registry.language_server_binary_statuses();
@@ -541,6 +549,7 @@ async fn test_extension_store_with_gleam_extension(cx: &mut TestAppContext) {
             node_runtime,
             language_registry.clone(),
             theme_registry.clone(),
+            slash_command_registry,
             cx,
         )
     });