Refine word completions (#26779)

Follow-up of https://github.com/zed-industries/zed/pull/26410

* Extract word completions into their own, `editor::ShowWordCompletions`
action so those could be triggered independently of completions
* Assign `ctrl-shift-space` binding to this new action
* Still keep words returned along the completions as in the original PR,
but:
* Tone down regular completions' fallback logic, skip words when the
language server responds with empty list of completions, but keep on
adding words if nothing or an error were returned instead
    * Adjust the defaults to wait for LSP completions infinitely
* Skip "words" with digits such as `0_usize` or `2.f32` from completion
items, unless a completion query has digits in it

Release Notes:

- N/A

crates/project/src/lsp_store.rs

@@ -4402,7 +4402,7 @@ impl LspStore {
         position: PointUtf16,
         context: CompletionContext,
         cx: &mut Context<Self>,
-    ) -> Task<Result<Vec<Completion>>> {
+    ) -> Task<Result<Option<Vec<Completion>>>> {
         let language_registry = self.languages.clone();
 
         if let Some((upstream_client, project_id)) = self.upstream_client() {
@@ -4430,7 +4430,7 @@ impl LspStore {
                 let mut result = Vec::new();
                 populate_labels_for_completions(completions, language, lsp_adapter, &mut result)
                     .await;
-                Ok(result)
+                Ok(Some(result))
             })
         } else if let Some(local) = self.as_local() {
             let snapshot = buffer.read(cx).snapshot();
@@ -4444,7 +4444,7 @@ impl LspStore {
             )
             .completions;
             if !completion_settings.lsp {
-                return Task::ready(Ok(Vec::new()));
+                return Task::ready(Ok(None));
             }
 
             let server_ids: Vec<_> = buffer.update(cx, |buffer, cx| {
@@ -4495,13 +4495,14 @@ impl LspStore {
                         ).fuse();
                         let new_task = cx.background_spawn(async move {
                             select_biased! {
-                                response = lsp_request => response,
+                                response = lsp_request => anyhow::Ok(Some(response?)),
                                 timeout_happened = timeout => {
                                     if timeout_happened {
                                         log::warn!("Fetching completions from server {server_id} timed out, timeout ms: {}", completion_settings.lsp_fetch_timeout_ms);
-                                        return anyhow::Ok(Vec::new())
+                                        Ok(None)
                                     } else {
-                                        lsp_request.await
+                                        let completions = lsp_request.await?;
+                                        Ok(Some(completions))
                                     }
                                 },
                             }
@@ -4510,9 +4511,11 @@ impl LspStore {
                     }
                 })?;
 
+                let mut has_completions_returned = false;
                 let mut completions = Vec::new();
                 for (lsp_adapter, task) in tasks {
-                    if let Ok(new_completions) = task.await {
+                    if let Ok(Some(new_completions)) = task.await {
+                        has_completions_returned = true;
                         populate_labels_for_completions(
                             new_completions,
                             language.clone(),
@@ -4522,8 +4525,11 @@ impl LspStore {
                         .await;
                     }
                 }
-
-                Ok(completions)
+                if has_completions_returned {
+                    Ok(Some(completions))
+                } else {
+                    Ok(None)
+                }
             })
         } else {
             Task::ready(Err(anyhow!("No upstream client or local language server")))