VR-Only Config Option

[hammy275]

Not sure why I haven't made this an option before, but this option (disabled by default) would shut off all ImmersiveMC features while out of VR when enabled.

A way I could implement this:

When ImmersiveMC disables itself
- Keeping this value in the ServerPlayerConfig, so the server knows to stop doing things. Could implement requests to get a player's config to return the default "disabled" config.
- Do all things done when leaving a world, then returning early at the top of render() and at the top of tick() if the player still isn't in VR (re-enabling everything if they hop into it).
- Load the "off"/"disabled" config.
- Special handling for the Mixins.

The above is WAY over-engineered. ImmersiveMC should already support enabling and disabling itself on the fly (pending #327 being fixed). Just disable it out of VR and re-enable entering VR.

Note that MC VR API can't detect whether the user is in VR or not on the title screen, so that will have to be addressed. That should come with hammy275/mc-vr-api#14

[hammy275]

Done! I think there's a small bug where the config change from OFF -> ON isn't acknowledged, but a game restart I believe fixes it.

Other than that, this is done!