Idea: CompletionItem.HelpLink

CompletionItem could have a `string HelpLink` property for a documentation URI. The type and name would be consistent with . Help could then render this as an OSC 8 hyperlink (), although doing so would complicate the width measurement during two-column layout and line breaking.

On the other hand, as long as class CompletionItem remains unsealed, this can be implemented in application code.

[CompletionItem in Language Server Protocol (LSP)](https://github.com/microsoft/language-server-protocol/blob/50732421db206ec7dd5c30b88326083bcaf826ef/_specifications/lsp/3.18/language/completion.md) does not appear to have this feature.