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/tabs_command.rs

@@ -47,7 +47,7 @@ impl SlashCommand for TabsSlashCommand {
     ) -> Task<Result<Vec<ArgumentCompletion>>> {
         let all_tabs_completion_item = if ALL_TABS_COMPLETION_ITEM.contains(&query) {
             Some(ArgumentCompletion {
-                label: ALL_TABS_COMPLETION_ITEM.to_owned(),
+                label: ALL_TABS_COMPLETION_ITEM.into(),
                 new_text: ALL_TABS_COMPLETION_ITEM.to_owned(),
                 run_command: true,
             })
@@ -63,7 +63,7 @@ impl SlashCommand for TabsSlashCommand {
                     .filter_map(|(path, ..)| {
                         let path_string = path.as_deref()?.to_string_lossy().to_string();
                         Some(ArgumentCompletion {
-                            label: path_string.clone(),
+                            label: path_string.clone().into(),
                             new_text: path_string,
                             run_command: true,
                         })