Flutter/Dart project detection improvement

[mikeborodin]

hi,

I ran into an issue that I couldn't start debugging. The error was about "dart:ui" inaccessible for this platform so I figured out it tries to run dart instead of flutter. For some reason I didn't have that .metadata file which is currently used to distinguish between pure dart and flutter projects. Maybe this logic could be improved? I'd be willing to contibute, I'm just a beginner in lua :)

  local is_flutter_project = vim.loop.fs_stat(path.join(root_dir, ".metadata"))