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/collab_ui/src/chat_panel/message_editor.rs

@@ -75,6 +75,17 @@ impl CompletionProvider for MessageEditorCompletionProvider {
     ) -> Task<Result<Option<language::Transaction>>> {
         Task::ready(Ok(None))
     }
+
+    fn is_completion_trigger(
+        &self,
+        _buffer: &Model<Buffer>,
+        _position: language::Anchor,
+        text: &str,
+        _trigger_in_words: bool,
+        _cx: &mut ViewContext<Editor>,
+    ) -> bool {
+        text == "@"
+    }
 }
 
 impl MessageEditor {