Further improve /tabs command and slash arguments completion (#16216)

* renames `/tabs` to `/tab`
* allows to insert multiple tabs when fuzzy matching by the names
* improve slash command completion API, introduce a notion of multiple
arguments
* properly fire off commands on arguments' completions with
`run_command: true`

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <marshall@zed.dev>

crates/assistant/src/slash_command/now_command.rs

@@ -32,7 +32,7 @@ impl SlashCommand for NowSlashCommand {
 
     fn complete_argument(
         self: Arc<Self>,
-        _query: String,
+        _arguments: &[String],
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
         _cx: &mut WindowContext,
@@ -42,7 +42,7 @@ impl SlashCommand for NowSlashCommand {
 
     fn run(
         self: Arc<Self>,
-        _argument: Option<&str>,
+        _arguments: &[String],
         _workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         _cx: &mut WindowContext,