lsp: Support insertReplace

Fixes #4473

helix-lsp/src/client.rs

@@ -311,6 +311,7 @@ impl Client {
                                     String::from("additionalTextEdits"),
                                 ],
                             }),
+                            insert_replace_support: Some(true),
                             ..Default::default()
                         }),
                         completion_item_kind: Some(lsp::CompletionItemKindCapability {

helix-term/src/ui/completion.rs

@@ -113,7 +113,8 @@ impl Completion {
                     let edit = match edit {
                         lsp::CompletionTextEdit::Edit(edit) => edit.clone(),
                         lsp::CompletionTextEdit::InsertAndReplace(item) => {
-                            unimplemented!("completion: insert_and_replace {:?}", item)
+                            // TODO: support using "insert" instead of "replace" via user config
+                            lsp::TextEdit::new(item.replace, item.new_text.clone())
                         }
                     };