Converted VR interface to OpenXR — BattleGroupVR Steam update #8615

Converted VR interface to OpenXR: This update switches out STEAMVR API in favor of OpenXR API to allow greater marketplace compatibility. Both Oculus App and SteamVR Launcher support OpenXR API. If the game does not detect your headset, make sure your Oculus App or SteamVR is up to date and for SteamVR users, that the OpenXR Runtime is set to SteamVR (SteamVR -> Settings -> Developer -> Set OpenXR Runtime). Oculus App usually defaults as OpenXR Runtime. Please let me know in the discord (https://discord.gg/4aCZ56W) if you run into any issues. Other changes: - Added "cease fire / open fire" and "reverse engines / forward engines" to voice commands. - Mission difficulty adjusted to exclude NPC ships when weighing your fleet for enemy difficulty calculation. - Changed cannon turret inaccuracy, smaller turrets are now slightly more accurate where large cannons are slightly less.