Find a way to prevent project to be selectable as Startup

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

I have search and found an abandoned ticket originally opened for Blazor solutions.
https://github.com/dotnet/aspnetcore/issues/21699

And it's true I'd really like the Interactive Client project that doesn't have a launchSettings.json not to appear in the selection.

But that's nothing compared to the aspire solution where only the last two projects are aspire projects with different configurations and which orchestrate all the other projects (so using launchSettings existence is not really an option).

### Describe the solution you'd like

Maybe the new solution format - slnx - has plan to address this topic
If not I would suggest adding a Start attribute on projects (maybe on the Folder) that should be in the start selection.
If no project have this attribute, fallback to the current way.

All in all the solution would look like this
```

...
```

In our case, all the aspire projects being in the same logical folder the Start attribute on the Folder would make sense
```

...
```

Of course having the IDE allowing to configure these options by right clicking on the projects/folders would be great

### Additional context

_No response_