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_manifest.rs

@@ -74,6 +74,8 @@ pub struct ExtensionManifest {
     pub grammars: BTreeMap<Arc<str>, GrammarManifestEntry>,
     #[serde(default)]
     pub language_servers: BTreeMap<LanguageServerName, LanguageServerManifestEntry>,
+    #[serde(default)]
+    pub slash_commands: BTreeMap<Arc<str>, SlashCommandManifestEntry>,
 }
 
 #[derive(Clone, Default, PartialEq, Eq, Debug, Deserialize, Serialize)]
@@ -128,6 +130,12 @@ impl LanguageServerManifestEntry {
     }
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct SlashCommandManifestEntry {
+    pub description: String,
+    pub requires_argument: bool,
+}
+
 impl ExtensionManifest {
     pub async fn load(fs: Arc<dyn Fs>, extension_dir: &Path) -> Result<Self> {
         let extension_name = extension_dir
@@ -190,5 +198,6 @@ fn manifest_from_old_manifest(
             .map(|grammar_name| (grammar_name, Default::default()))
             .collect(),
         language_servers: Default::default(),
+        slash_commands: BTreeMap::default(),
     }
 }