Problem: values from completionList.itemDefaults always replace the
item's own commitCharacters and data. Since 3.18 a server can ask for
them to be merged instead.
Solution: Handle CompletionList.applyKind and advertise applyKindSupport.
Drop itemDefaults once folded into the items: get_items() runs twice on
the same result and a Merge isn't idempotent.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#completionItemApplyKinds
9fae7234f6