[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.

首先阅读 plugin.lua，并追踪如何将 plugin 提供的信息传递到 language server 的 diagnostics 中。确认范围检查的预期行为，并定义测试以证明来自 plugin 的自定义 diagnostics 能够到达用户。