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"))

[akinsho]

A contribution (regardless of lua skill level, it's not a complex project) would be very welcome as I don't have any time to do any work on this at all at the moment.