Merge /active command into /tabs one (#16154)

Now, tabs have arguments, `active` (default, applied also for no
arguments case) and `all` to insert the active tab only or all tabs.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -17,7 +17,6 @@ use std::{
 use ui::ActiveTheme;
 use workspace::Workspace;
 
-pub mod active_command;
 pub mod default_command;
 pub mod diagnostics_command;
 pub mod docs_command;