HMR for _app can fail to trigger when added after starting dev server

[ijjk]

When adding an _app file after starting the dev server (next dev) HMR can fail to trigger in some cases like when an error is added in _app (a syntax error or missing import) and then the error is fixed.

We need to ensure the HMR behaves correctly regardless of if _app was present before or after starting the dev server.

This might be related to the new fallback handling we're leveraging in webpack 5 that was added in #28279

[ijjk]

Closing as this should be resolved now and we have test cases covering _app errors and adding/removing

[github-actions]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.