Add slash commands for adding context into the assistant (#12102)

Tasks

* [x] remove old flaps and output when editing a slash command
* [x] the completing a command name that takes args, insert a space to
prepare for typing an arg
* [x] always trigger completions when  typing in a slash command
* [x] don't show line numbers
* [x] implement `prompt` command
* [x] `current-file` command
* [x] state gets corrupted on `duplicate line up` on a slash command
* [x] exclude slash command source from completion request

Next steps:
* show output token count in flap trailer
* add `/project` command that matches project ambient context
* delete ambient context

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/gpui/src/window.rs

@@ -4443,6 +4443,9 @@ impl<V: 'static> From<WindowHandle<V>> for AnyWindowHandle {
     }
 }
 
+unsafe impl<V> Send for WindowHandle<V> {}
+unsafe impl<V> Sync for WindowHandle<V> {}
+
 /// A handle to a window with any root view type, which can be downcast to a window with a specific root view type.
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct AnyWindowHandle {