use `@Accessor` over accesswidener and more static fields

[NicBOMB]

2 examples of mixin extras usage are in MinecraftVRMixin and OptionsVRMixin respectively. Cleanup of OptionsVRMixin also revealed the keymapping category injection could be separated, so KeyMappingVRMixin was added and the accesswidener reduced. This implementation benefits the VivecraftVRMod class greatly, as it no longer needs an INSTANCE field at all. All fields may be initialized statically. This may also avoid an injection priority conflict with fabric according to @fayer3 .