Disable clangd's inactiveRegions support (#26539)

Disables https://github.com/zed-industries/zed/pull/26146 until a better
way to add diagnostics is found.
Overall, the PR had made changes that are worth keeping instead of
reverting, such as finally extracting out r-a's language server logic
into an `_ext.rs` file.

Release Notes:

- N/A

crates/project/src/lsp_store/clangd_ext.rs

@@ -35,6 +35,10 @@ pub fn register_notifications(
     }
     let server_id = language_server.server_id();
 
+    // TODO: inactiveRegions support needs do add diagnostics, not replace them as `this.update_diagnostics` call below does
+    if true {
+        return;
+    }
     language_server
         .on_notification::<InactiveRegions, _>({
             let adapter = adapter.clone();