Fix force-start crash (as a host)

Reproduction:

You are host
Player2 is not ready
As host, click the 'Ready' button multiple times in briefing to show the force-start confirmation menu
You are marked as ready now
Player2 now clicks ready
Game starts
The force-start menu is still open
If you click "Confirm" now, the game will crash because it didn't expect to be in-game while forcing the start

Application has crashed: C++ exception
mods/VanillaHUD Plus/lua/Scripts.lua:57: attempt to call method 'start_game_intro' (a nil value)

SCRIPT STACK

callback_func() @mods/base/req/utils/QuickMenu.lua:58
button_pressed() lib/managers/dialogs/dialog.lua:61
button_pressed() lib/managers/dialogs/genericdialog.lua:375
button_pressed_callback() lib/managers/dialogs/genericdialog.lua:371
mouse_press() lib/managers/dialogs/genericdialog.lua:123
lib/managers/mousepointermanager.lua:345

https://github.com/steam-test1/VPlusHUD/blob/b8d57176d4b1c81f91f16421b75d7e8689cb1168/devlua/Scripts.lua

Changes:

Avoid the crash by checking if we are in the correct game state
Refactor the function, it's in a separate file now

steam-test1 / VPlusHUD

Fix force-start crash (as a host) #283