feat(lsp): support completion itemDefaults

runtime/doc/news.txt

@@ -125,6 +125,15 @@ The following changes may require adaptations in user config or plugins.
 • Returning any truthy value from a callback passed to |nvim_create_autocmd()|
   (rather than just `true`) will delete the autocommand.
 
+• |vim.lsp.util.extract_completion_items()| will no longer return reliable
+  results, since it does not apply `itemDefaults` when its input is a
+  `CompletionList`.
+  Moreover, since support for LSP `completionList.itemDefaults` was added,
+  some third party plugins might be negatively impacted in case the language
+  servers support the feature but the plugin does not.
+  If necessary, the respective capability can be
+  removed when calling |vim.lsp.protocol.make_client_capabilities()|.
+
 ==============================================================================
 BREAKING CHANGES IN HEAD                                    *news-breaking-dev*
 
@@ -213,6 +222,11 @@ The following new APIs and features were added.
     the original LSP `Location` or `LocationLink`.
   • Added support for connecting to servers using named pipes (Windows) or
     unix domain sockets (Unix) via |vim.lsp.rpc.domain_socket_connect()|.
+  • Added support for `completionList.itemDefaults`, reducing overhead when
+    computing completion items where properties often share the same value
+    (e.g. `commitCharacters`). Note that this might affect plugins and
+    language servers that don't support the feature, and in such cases the
+    respective capability can be unset.
 
 • Treesitter
   • Bundled parsers and queries (highlight, folds) for Markdown, Python, and