Cancel language server requests if request is dropped

Before this change, we would send requests to language servers without
canceling them even if we never wait for their response.

Example: when doing document highlights, we'd send a request (modulo
debouncing) whenever a change was made, ignoring previously sent
requests that might still be in-flight.

With this change, we now send a Cancel request (from the LSP spec) to
the language server in case no one listens to the response anymore
(which is what happens when the `Future` returned by `request_internal`)
is dropped.

crates/util/src/util.rs

@@ -299,7 +299,7 @@ pub struct Deferred<F: FnOnce()>(Option<F>);
 
 impl<F: FnOnce()> Deferred<F> {
     /// Drop without running the deferred function.
-    pub fn cancel(mut self) {
+    pub fn abort(mut self) {
         self.0.take();
     }
 }