Grouping: Improve grouping for App Hangs only having one main in-app frame

A customer in https://github.com/getsentry/sentry-cocoa/issues/6345 and also in an internal customer case in [Notion](https://www.notion.so/sentry/2838b10e4b5d8055a6face5d727dfb73) both reported that app hangs only having one in-app frame with `main` are grouped into the same group, which makes it really hard to debug them.

While https://github.com/getsentry/sentry-cocoa/issues/6345 surfaced that most of these app hangs stem from false positives, which we are going to fix with https://github.com/getsentry/sentry-cocoa/pull/6381, it still can occur that the app hangs with only one in-app frame that is `main`. In that case we must group differently. One idea is to group by all frames for app hangs when they only have one in app frame that is main.