Allow using different flutter binaries for the sdk and flutter itself

[ArtixBTW]

On NixOS using a different flutter binary is needed or else commands like :FlutterRun won't work as the flutter binary that's with dart-sdk is different from the "wrapped" flutter in $PATH this "wrapped" flutter simply runs flutter with a lot of required dependencies. I know this similar to #259 which solves the same problem in a different way. But if this implementation isn't to your liking, I've been also been thinking of an alternative solution where dart_bin, dart_sdk, flutter_sdk, and flutter_bin are all possible with the existing system replaced (using flutter_bin to find the others if they aren't supplied, similar to how it is now). If I'm correct those options on their own should be enough to account for every possible scenario and close all further requests for custom paths.