Support detecting `Custom Flutter version` from `fvm`

[TahaTesser]

I currently use only Custom Flutter Version in fvm for forked Flutter SDK and I've also set it as active and global Flutter in fvm yet Sidekick doesn't detect this.

tahatesser@Tahas-MacBook-Pro flutter % fvm list
Cache Directory:  /Users/tahatesser/Code

.fvm
flutter (active) (global)
android-sdk
tahatesser@Tahas-MacBook-Pro flutter % 

I cannot use the SDK installed by Sidekick for my use case.