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_slash_command/src/assistant_slash_command.rs

@@ -37,7 +37,7 @@ pub trait SlashCommand: 'static + Send + Sync {
         query: String,
         cancel: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
-        cx: &mut AppContext,
+        cx: &mut WindowContext,
     ) -> Task<Result<Vec<ArgumentCompletion>>>;
     fn requires_argument(&self) -> bool;
     fn run(