Some users have their Xcode settings set to not debug (see example here https://github.com/flutter/flutter/issues/136197#issuecomment-1766834195). This will cause the [engine check for a debugger](https://github.com/flutter/engine/blob/22ce5c6a45e2898b4ce348c514b5fa42ca25bc88/runtime/ptrace_check.cc#L56-L71) to fail, which will cause an error and cause the app to crash.

This PR parses the scheme file to ensure the scheme is set to start a debugger and warn the user if it's not.

Fixes https://github.com/flutter/flutter/issues/136197.