[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.

首先閱讀 plugin.lua，並追蹤如何將 plugin 提供的資訊傳遞至 language server 的 diagnostics。確認範圍檢查的預期行為，並定義測試以證明來自 plugin 的自訂 diagnostics 能夠傳達給使用者。