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/search_command.rs

@@ -49,7 +49,7 @@ impl SlashCommand for SearchSlashCommand {
 
     fn complete_argument(
         self: Arc<Self>,
-        _query: String,
+        _arguments: &[String],
         _cancel: Arc<AtomicBool>,
         _workspace: Option<WeakView<Workspace>>,
         _cx: &mut WindowContext,
@@ -59,7 +59,7 @@ impl SlashCommand for SearchSlashCommand {
 
     fn run(
         self: Arc<Self>,
-        argument: Option<&str>,
+        arguments: &[String],
         workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
@@ -67,13 +67,13 @@ impl SlashCommand for SearchSlashCommand {
         let Some(workspace) = workspace.upgrade() else {
             return Task::ready(Err(anyhow::anyhow!("workspace was dropped")));
         };
-        let Some(argument) = argument else {
+        if arguments.is_empty() {
             return Task::ready(Err(anyhow::anyhow!("missing search query")));
         };
 
         let mut limit = None;
         let mut query = String::new();
-        for part in argument.split(' ') {
+        for part in arguments {
             if let Some(parameter) = part.strip_prefix("--") {
                 if let Ok(count) = parameter.parse::<usize>() {
                     limit = Some(count);