Allow LSP adapters to decide, which diagnostics to underline (#31450)

Closes
https://github.com/zed-industries/zed/pull/31355#issuecomment-2910439798

<img width="1728" alt="image"
src="https://github.com/user-attachments/assets/2eaa8e9b-00bc-4e99-ac09-fceb2d932e41"
/>


Release Notes:

- N/A

crates/proto/proto/buffer.proto

@@ -261,6 +261,7 @@ message Diagnostic {
 
     bool is_disk_based = 10;
     bool is_unnecessary = 11;
+    bool underline = 15;
 
     enum Severity {
         None = 0;