Use POSIX path in `.vscode/settings.json`

[jibbers42]

Is your feature request related to a problem? Please describe. FVM uses OS/filesystem specific path separators in .vscode/settings.json for the setting dart.flutterSdkPath. For teams that share settings, this will causes git churn and perhaps not even work on Mac/Linux after a Windows user changes the flutter version.

Describe the solution you'd like FVM should always write a POSIX path no matter what OS is in use. For example, on Windows write: "dart.flutterSdkPath": ".fvm/flutter_sdk", instead of: "dart.flutterSdkPath": ".fvm\\versions\\3.16.9",

I've tested this on Windows and it seems to work fine, see discussion https://github.com/leoafarias/fvm/discussions/634.