Allow fvm config files as source for flutter-version-file

[ueman]

It would be nice if you could use fvm version files as source for the flutter-version-file parameter.

[kuhnroyal]

I created #101 3 years ago, maybe this should be reconsiderd now that fvm is used widely.

[bartekpacia]

Hi @ueman and @kuhnroyal 👋🏻

I think it's a great idea! I just would like to make sure we do it right. I am myself not a heavy user of fvm – I only do fvm install x.y.z and then fvm global x.y.z. I know there's both fvm_config.json and sth like .fvvmrc since 3.0. Not sure what's the distinction/use cases and differences.

Feel free to make a PR that takes the above into account and updates the README accordingly.

[kuhnroyal]

The 2 config files are basically the same. In FVM 3 the config file has just been separated from the .fvm SDK link.
A problem that comes to mind is the extra flavor config in FVM which will be hard to pass as an argument.

[dkbast]

When adding support for fvm it would be great if there would also be an fvm symlink so that we can use fvm flutter commands in our actions (e.g. using a steps from a Makefile)