Lua Menu context

We need a way to run Lua scripts in the main menu, for the following reasons:

1. as we move more and more settings into Lua, it also becomes impossible to edit them before loading the game
2. in multiplayer, other types of scripts can come from the server and be unsafe. menu scripts are planned to only be installed by the player, which makes them ideal for sensitive APIs like #7648 (closed)
3. eventually, fully dehardcoding main menu is useful for supporting new games made in OpenMW
4. "meta" mods that affect multiple saves might want to perform actions on start, or add new UI to the main menu