feat: fvm support

[tomassasovsky]

Using fvm I specify the flutter version to be used per-directory basis. It may mean that I don't have flutter available globally, or it can link to a different version of flutter than globally accessible. This would be the version that I'd like to use when resolving dependencies for a given project.

Some tools (like pubviz or bash scripts using flutter internally) require additional steps to make this happen, like overriding flutter for the scope of a program, or temporarily changing flutter to link to the desired version, or sometimes even modifying the script itself. Being able to pass: very_good packages get --fluter ~/fvm/versions/beta/bin/ could be really handy.

Originally posted by @orestesgaolin in https://github.com/VeryGoodOpenSource/very_good_cli/issues/174#issuecomment-908513565