Allow user to save favorites playlist to local disk

[unsupervisednn]

I got a different VR headset and lost all my favorites songs list. I want to gauge if this is a feature that is likely to be merged if I work on it. I know the leaderboard is using firebase for persistence. Favorites could probably be stored there but the leaderboard hasn't been loading pointing to firebase overload, so I don't want to rely on that.

~~To load/restore the favorites file I might have to implement a file chooser webxr UI as an external lib and link it in~~ Edit: Apparently browser file system api is a built in dialog.

cc @dmarcos

[dmarcos]

I'm good if it can be done simply and without any backend or user management to maintain.

[frabul]

One way to do it would be to add export and import buttons. They can be imported or exported from localStorage in json format.

[kenshin171]

Similiar to my feature request here https://github.com/supermedium/moonrider/issues/171