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/file_finder/src/open_path_prompt.rs

@@ -1,7 +1,7 @@
 use futures::channel::oneshot;
 use fuzzy::StringMatchCandidate;
 use picker::{Picker, PickerDelegate};
-use project::{compare_paths, DirectoryLister};
+use project::DirectoryLister;
 use std::{
     path::{Path, PathBuf},
     sync::{
@@ -11,7 +11,7 @@ use std::{
 };
 use ui::{prelude::*, LabelLike, ListItemSpacing};
 use ui::{ListItem, ViewContext};
-use util::maybe;
+use util::{maybe, paths::compare_paths};
 use workspace::Workspace;
 
 pub(crate) struct OpenPathPrompt;