Re-filter existing completions on selection update

We still request new completions, but this ensures results are up-to-date in the meantime.

Also: Cancel any pending completions task when we dismiss the completions dialog or start a new completions request.
Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/editor/src/multi_buffer/anchor.rs

@@ -106,7 +106,7 @@ impl Anchor {
 }
 
 impl ToOffset for Anchor {
-    fn to_offset<'a>(&self, snapshot: &MultiBufferSnapshot) -> usize {
+    fn to_offset(&self, snapshot: &MultiBufferSnapshot) -> usize {
         self.summary(snapshot)
     }
 }