Add a completion handler type in helix-view for tracking responses

This will replace the `Sender<CompletionEvent>` in the child commits.
It tracks sender alongside extra metadata about the responses received
from providers - namely whether a request is incomplete or not - which
can be reused between subsequent requests to the provider.

helix-view/src/handlers/completion.rs

@@ -1,4 +1,41 @@
-use crate::{DocumentId, ViewId};
+use std::{collections::HashMap, sync::Arc};
+
+use helix_core::completion::CompletionProvider;
+use helix_event::{send_blocking, TaskController};
+
+use crate::{document::SavePoint, DocumentId, ViewId};
+
+use tokio::sync::mpsc::Sender;
+
+pub struct CompletionHandler {
+    event_tx: Sender<CompletionEvent>,
+    pub active_completions: HashMap<CompletionProvider, ResponseContext>,
+    pub request_controller: TaskController,
+}
+
+impl CompletionHandler {
+    pub fn new(event_tx: Sender<CompletionEvent>) -> Self {
+        Self {
+            event_tx,
+            active_completions: HashMap::new(),
+            request_controller: TaskController::new(),
+        }
+    }
+
+    pub fn event(&self, event: CompletionEvent) {
+        send_blocking(&self.event_tx, event);
+    }
+}
+
+pub struct ResponseContext {
+    /// Whether the completion response is marked as "incomplete."
+    ///
+    /// This is used by LSP. When completions are "incomplete" and you continue typing, the
+    /// completions should be recomputed by the server instead of filtered.
+    pub is_incomplete: bool,
+    pub priority: i8,
+    pub savepoint: Arc<SavePoint>,
+}
 
 pub enum CompletionEvent {
     /// Auto completion was triggered by typing a word char