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/slash_command.rs

@@ -217,6 +217,7 @@ impl CompletionProvider for SlashCommandCompletionProvider {
         &self,
         buffer: &Model<Buffer>,
         buffer_position: Anchor,
+        _: editor::CompletionContext,
         cx: &mut ViewContext<Editor>,
     ) -> Task<Result<Vec<project::Completion>>> {
         let Some((name, argument, command_range, argument_range)) =