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/fuzzy/src/matcher.rs

@@ -445,6 +445,7 @@ mod tests {
             let lowercase_path = path.to_lowercase().chars().collect::<Vec<_>>();
             let char_bag = CharBag::from(lowercase_path.as_slice());
             path_entries.push(PathMatchCandidate {
+                is_dir: false,
                 char_bag,
                 path: &path_arcs[i],
             });
@@ -468,6 +469,7 @@ mod tests {
                 path: Arc::from(candidate.path),
                 path_prefix: "".into(),
                 distance_to_relative_ancestor: usize::MAX,
+                is_dir: false,
             },
         );