Improve /tabs completion workflow (#16168)

Follow-up of https://github.com/zed-industries/zed/pull/16154

Reworks /tabs arguments to allow:
* current tab by default, if no arguments are present
* fuzzy-matching over paths of the related tabs
* `all` case to insert all tabs at once

Release Notes:

- N/A

crates/assistant/src/slash_command/file_command.rs

@@ -104,7 +104,7 @@ impl SlashCommand for FileSlashCommand {
         query: String,
         cancellation_flag: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
-        cx: &mut AppContext,
+        cx: &mut WindowContext,
     ) -> Task<Result<Vec<ArgumentCompletion>>> {
         let Some(workspace) = workspace.and_then(|workspace| workspace.upgrade()) else {
             return Task::ready(Err(anyhow!("workspace was dropped")));