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/language/src/language.rs

@@ -1560,6 +1560,22 @@ impl CodeLabel {
             self.runs.push((start_ix..end_ix, highlight));
         }
     }
+
+    pub fn text(&self) -> &str {
+        self.text.as_str()
+    }
+}
+
+impl From<String> for CodeLabel {
+    fn from(value: String) -> Self {
+        Self::plain(value, None)
+    }
+}
+
+impl From<&str> for CodeLabel {
+    fn from(value: &str) -> Self {
+        Self::plain(value.to_string(), None)
+    }
 }
 
 impl Ord for LanguageMatcher {