lsp: Provide completion reason in the request (#12893)

This should help LS make a better call about the completions it should
return back to the caller. For example, it speeds up import completions
for typescript.
Before: 


https://github.com/zed-industries/zed/assets/24362066/b38fd565-f9ff-4db7-a87f-c3b31a9fdc96

after: 


https://github.com/zed-industries/zed/assets/24362066/d4fbc9ae-9aab-4543-b9f6-16acf1619576


This should be merged after 06.12 Preview to give it some time on
Nightly.

Release Notes:

- N/A

crates/assistant/src/assistant_panel.rs

@@ -2350,7 +2350,7 @@ impl ContextEditor {
                     editor.insert(&format!("/{name}"), cx);
                     if command.requires_argument() {
                         editor.insert(" ", cx);
-                        editor.show_completions(&ShowCompletions, cx);
+                        editor.show_completions(&ShowCompletions::default(), cx);
                     }
                 });
             });