[Feature Request]: Way to add custom diagnostics in plugin.lua

It would be useful to be able to add custom diagnostics that can be propagated from plugin.lua in a custom plugin.

For a plugin my team is developing, we need a way to check that values the user is setting a variable to fall within a certain range (among other possible diagnostics). I can understand why this wouldn't be desirable to add to the core diagnostics.

まず plugin.lua を読み、plugin が提供する情報を language server の diagnostics に伝播させる方法を追跡します。range checks で期待される動作を確認し、plugin からのカスタム diagnostics がユーザーに届くことを示すテストを定義します。