Player dual wielding support

[Blixibon]

This PR allows the player to use dual pistols in the same way that npc_assassin can. For the time being, this can only be initiated via a new CreateLeftHandGun input. Dual wielding generally allows the player to fire their weapon twice as fast with twice as much ammo, but the weapon also becomes twice as inaccurate over time. Dual wielded weapons can also (optionally) use their own viewmodel or icons specified in the weapon script.

This PR currently only integrates player dual wielding into weapon_pistol and weapon_pulsepistol.

[1upD]

I left two questions I'd like to address before approving the review