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_api/src/extension_api.rs

@@ -24,6 +24,7 @@ pub use wit::{
         npm_package_latest_version,
     },
     zed::extension::platform::{current_platform, Architecture, Os},
+    zed::extension::slash_command::SlashCommand,
     CodeLabel, CodeLabelSpan, CodeLabelSpanLiteral, Command, DownloadedFileType, EnvVars,
     LanguageServerInstallationStatus, Range, Worktree,
 };
@@ -104,6 +105,16 @@ pub trait Extension: Send + Sync {
     ) -> Option<CodeLabel> {
         None
     }
+
+    /// Runs the given slash command.
+    fn run_slash_command(
+        &self,
+        _command: SlashCommand,
+        _argument: Option<String>,
+        _worktree: &Worktree,
+    ) -> Result<Option<String>, String> {
+        Ok(None)
+    }
 }
 
 /// Registers the provided type as a Zed extension.
@@ -139,6 +150,8 @@ static mut EXTENSION: Option<Box<dyn Extension>> = None;
 pub static ZED_API_VERSION: [u8; 6] = *include_bytes!(concat!(env!("OUT_DIR"), "/version_bytes"));
 
 mod wit {
+    #![allow(clippy::too_many_arguments)]
+
     wit_bindgen::generate!({
         skip: ["init-extension"],
         path: "./wit/since_v0.0.7",
@@ -209,6 +222,14 @@ impl wit::Guest for Component {
         }
         Ok(labels)
     }
+
+    fn run_slash_command(
+        command: SlashCommand,
+        argument: Option<String>,
+        worktree: &Worktree,
+    ) -> Result<Option<String>, String> {
+        extension().run_slash_command(command, argument, worktree)
+    }
 }
 
 /// The ID of a language server.