Allow completing slash command arguments from extensions (#13240)

This PR extends the extension API with support for completing slash
command arguments for slash commands defined in extensions.

Release Notes:

- N/A

crates/extension_api/src/extension_api.rs

@@ -108,7 +108,16 @@ pub trait Extension: Send + Sync {
         None
     }
 
-    /// Runs the given slash command.
+    /// Returns the completions that should be shown when completing the provided slash command with the given query.
+    fn complete_slash_command_argument(
+        &self,
+        _command: SlashCommand,
+        _query: String,
+    ) -> Result<Vec<String>, String> {
+        Ok(Vec::new())
+    }
+
+    /// Returns the output from running the provided slash command.
     fn run_slash_command(
         &self,
         _command: SlashCommand,
@@ -225,6 +234,13 @@ impl wit::Guest for Component {
         Ok(labels)
     }
 
+    fn complete_slash_command_argument(
+        command: SlashCommand,
+        query: String,
+    ) -> Result<Vec<String>, String> {
+        extension().complete_slash_command_argument(command, query)
+    }
+
     fn run_slash_command(
         command: SlashCommand,
         argument: Option<String>,

crates/extension_api/wit/since_v0.0.7/extension.wit

@@ -122,6 +122,9 @@ world extension {
     export labels-for-completions: func(language-server-id: string, completions: list<completion>) -> result<list<option<code-label>>, string>;
     export labels-for-symbols: func(language-server-id: string, symbols: list<symbol>) -> result<list<option<code-label>>, string>;
 
-    /// Runs the provided slash command.
+    /// Returns the completions that should be shown when completing the provided slash command with the given query.
+    export complete-slash-command-argument: func(command: slash-command, query: string) -> result<list<string>, string>;
+
+    /// Returns the output from running the provided slash command.
     export run-slash-command: func(command: slash-command, argument: option<string>, worktree: borrow<worktree>) -> result<slash-command-output, string>;
 }