Extract ExtensionSlashCommand to assistant_slash_command crate (#20617)

This PR extracts the `ExtensionSlashCommand` implementation to the
`assistant_slash_command` crate.

The slash command related methods have been added to the `Extension`
trait. We also create separate data types for the slash command data
within the `extension` crate so that we can talk about them without
depending on the `extension_host` or `assistant_slash_command`.

Release Notes:

- N/A

crates/extension/src/extension.rs

@@ -1,5 +1,6 @@
 pub mod extension_builder;
 mod extension_manifest;
+mod slash_command;
 
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
@@ -10,6 +11,7 @@ use gpui::Task;
 use semantic_version::SemanticVersion;
 
 pub use crate::extension_manifest::*;
+pub use crate::slash_command::*;
 
 #[async_trait]
 pub trait WorktreeDelegate: Send + Sync + 'static {
@@ -32,6 +34,19 @@ pub trait Extension: Send + Sync + 'static {
     /// Returns the path to this extension's working directory.
     fn work_dir(&self) -> Arc<Path>;
 
+    async fn complete_slash_command_argument(
+        &self,
+        command: SlashCommand,
+        arguments: Vec<String>,
+    ) -> Result<Vec<SlashCommandArgumentCompletion>>;
+
+    async fn run_slash_command(
+        &self,
+        command: SlashCommand,
+        arguments: Vec<String>,
+        resource: Option<Arc<dyn WorktreeDelegate>>,
+    ) -> Result<SlashCommandOutput>;
+
     async fn suggest_docs_packages(&self, provider: Arc<str>) -> Result<Vec<String>>;
 
     async fn index_docs(