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/extension/src/extension_slash_command.rs

@@ -63,7 +63,7 @@ impl SlashCommand for ExtensionSlashCommand {
                                 completions
                                     .into_iter()
                                     .map(|completion| ArgumentCompletion {
-                                        label: completion.label,
+                                        label: completion.label.into(),
                                         new_text: completion.new_text,
                                         run_command: completion.run_command,
                                     })