Allow symlinking similarly to FVM.

[gaetschwartz]

In fvm, when you use a version, it creates a symlink to your local flutter version at ./.fvm/flutter_sdk to which you can then point in your .vscode/settings.json. It is pretty useful as it allows to stage the .vscode/settings.json file as it doesn't contain an absolute path, but a relative one (./.fvm/flutter_sdk). On the otherhand, puro sets the paths to an absolute, machine dependant path. That would be great to allow a behavior similar to FVM's in puro :)