zed_extension_api: Return structured slash command completions (#13879)

This PR updates the extension API to use structured slash command
completions instead of plain strings.

This allows slash commands defined in extensions to take advantage of
the improvements made in #13876.

Release Notes:

- N/A

crates/extension_api/wit/since_v0.0.7/slash-command.wit

@@ -28,4 +28,14 @@ interface slash-command {
         /// The label to display in the placeholder for this section.
         label: string,
     }
+
+    /// A completion for a slash command argument.
+    record slash-command-argument-completion {
+        /// The label to display for this completion.
+        label: string,
+        /// The new text that should be inserted into the command when this completion is accepted.
+        new-text: string,
+        /// Whether the command should be run when accepting this completion.
+        run-command: bool,
+    }
 }