Sort completions according to LSP's sortText

When typing "someobject." there may be many valid completions.
Some servers such as rust-analyzer send the sortText property
which prescribes a completion ordering, usually to put
commonly-used methods first.

Let's use the proposed Kakoune patch to implement this ordering.
Note that as soon as the user has typed a few characters, the fuzzy
matching score will dominate any difference in ordering.

src/language_features/completion.rs

@@ -51,13 +51,17 @@ pub fn text_document_completion(meta: EditorMeta, params: EditorParams, ctx: &mu
     );
 }
 
+fn sort_text(item: &CompletionItem) -> &str {
+    item.sort_text.as_ref().unwrap_or(&item.label)
+}
+
 fn editor_completion(
     meta: EditorMeta,
     params: TextDocumentCompletionParams,
     results: Vec<(String, Option<CompletionResponse>)>,
     ctx: &mut Context,
 ) {
-    let items = results
+    let mut items: Vec<(String, CompletionItem)> = results
         .into_iter()
         .flat_map(|(server_name, items)| {
             let items = match items {
@@ -70,6 +74,11 @@ fn editor_completion(
         })
         .collect();
 
+    // TODO Group by server?
+    items.sort_by(|(_left_server, left), (_right_server, right)| {
+        sort_text(left).cmp(sort_text(right))
+    });
+
     let version = meta.version;
     ctx.completion_items = items;
     ctx.completion_items_timestamp = version;