Add nemesis mode + 'convar'

[Frozen-H2O]

• Add convar zr_nemesis_mode to disable infections during EZRRoundState::POST_INFECTION. Zombies knifing instead instantly kills CTs, does not swap their team, and leaves them dead. Dead zombie respawning will still be controlled as normal with g_bRespawnEnabled.
• Make it so if zr_nemesis_mode and mp_respawn_on_death_ct are both true, do not end the round when all CTs die (as they will still respawn as CT). Ignore the T respawning cvar, as T respawning is controlled by g_bRespawnEnabled.