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/src/extension_api.rs

@@ -25,7 +25,9 @@ pub use wit::{
         npm_package_latest_version,
     },
     zed::extension::platform::{current_platform, Architecture, Os},
-    zed::extension::slash_command::{SlashCommand, SlashCommandOutput, SlashCommandOutputSection},
+    zed::extension::slash_command::{
+        SlashCommand, SlashCommandArgumentCompletion, SlashCommandOutput, SlashCommandOutputSection,
+    },
     CodeLabel, CodeLabelSpan, CodeLabelSpanLiteral, Command, DownloadedFileType, EnvVars,
     KeyValueStore, LanguageServerInstallationStatus, Range, Worktree,
 };
@@ -114,7 +116,7 @@ pub trait Extension: Send + Sync {
         &self,
         _command: SlashCommand,
         _query: String,
-    ) -> Result<Vec<String>, String> {
+    ) -> Result<Vec<SlashCommandArgumentCompletion>, String> {
         Ok(Vec::new())
     }
 
@@ -247,7 +249,7 @@ impl wit::Guest for Component {
     fn complete_slash_command_argument(
         command: SlashCommand,
         query: String,
-    ) -> Result<Vec<String>, String> {
+    ) -> Result<Vec<SlashCommandArgumentCompletion>, String> {
         extension().complete_slash_command_argument(command, query)
     }