fix: lsp and diagnostic highlight priority (#17461)

Closes https://github.com/neovim/neovim/issues/17456

* treesitter uses the default highlight priority of 50
* diagnostic highlights have a priority of 150
* lsp reference highlights have a priority of 200

This ensures proper ordering.

runtime/lua/vim/diagnostic.lua

@@ -920,7 +920,10 @@ M.handlers.underline = {
         underline_ns,
         higroup,
         { diagnostic.lnum, diagnostic.col },
-        { diagnostic.end_lnum, diagnostic.end_col }
+        { diagnostic.end_lnum, diagnostic.end_col },
+        'v',
+        false,
+        150
       )
     end
     save_extmarks(underline_ns, bufnr)