Support bin/__project_name__ as a default entry point #4906

Right now if you press F5 in a Dart CLI script we'll detect bin/main.dart as a default entry point. However, dart create has created the bin file with the same name as the project for a while now. This results in us telling you to add a program to your launch.json even though a) we could trivially infer this if it matches the folder name and b) we generally don't want people to create launch.jsons for trivial projects.