Improve slash commands (#16195)

This PR:

- Makes slash commands easier to compose by adding a concept,
`CompletionIntent`. When using `tab` on a completion in the assistant
panel, that completion item will be expanded but the associated command
will not be run. Using `enter` will still either run the completion item
or continue command composition as before.
- Fixes a bug where running `/diagnostics` on a project with no
diagnostics will delete the entire command, rather than rendering an
empty header.
- Improves the autocomplete rendering for files, showing when
directories are selected and re-arranging the results to have the file
name or trailing directory show first.

<img width="642" alt="Screenshot 2024-08-13 at 8 12 43 PM"
src="https://github.com/user-attachments/assets/97c96cd2-741f-4f15-ad03-7cf78129a71c">


Release Notes:

- N/A

crates/assistant/src/slash_command/terminal_command.rs

@@ -48,7 +48,7 @@ impl SlashCommand for TerminalSlashCommand {
         _cx: &mut WindowContext,
     ) -> Task<Result<Vec<ArgumentCompletion>>> {
         Task::ready(Ok(vec![ArgumentCompletion {
-            label: LINE_COUNT_ARG.to_string(),
+            label: LINE_COUNT_ARG.into(),
             new_text: LINE_COUNT_ARG.to_string(),
             run_command: true,
         }]))